The study abstract states:

Participants whose partisanship matched that of the candidate they were evaluating consistently rated the lightened photographs as more representative of the candidate than the darkened photographs, whereas participants whose partisanship did not match that of the candidate showed the opposite pattern. For evaluations of Barack Obama, the extent to which people rated lightened photographs as representative of him was positively correlated with their stated voting intentions and reported voting behavior in the 2008 Presidential election. This effect persisted when controlling for political ideology and racial attitudes. These results suggest that people's visual representations of others are related to their own preexisting beliefs and to the decisions they make in a consequential context.

Ben Smith at the Politico isn't entirely sold on the study's conclusion, noting:

The study seems to indicate in passing that the race of the participants doesn't affect the outcome, though it isn't totally clear on that point. It also seems to buy in to the claim that Hillary Clinton artificially darkened an image of Obama, which wasn't terribly widely believed.

Anyway, the research's most practical finding seems to be that devious political hacks don't need to play games with candidates' pictures because the voters are doing it themselves:

Although the number of Blacks holding public office has increased dramatically over the years, light-skinned Blacks have consistently been over-represented, and dark-skinned Blacks consistently under-represented, as elected officials (26). Some have even suggested that a successful strategy for Black candidates who are running for office would be to look ‘‘more white'' in appearance... Our results suggest that voters themselves may alter how they see a racially ambiguous candidate, depending on their own level of support and their corresponding desire to see the candidate favorably.

Smith seems to leave off two other large instances of darkening that occurred in the past 15 years. The first, one that made political headlines, was the darkening of Harold Ford, Jr. in the 2006 Senate race in Tennessee. As Time magazine reported:

[A]s the race has heated up, the issue of race itself has become an ugly part of the campaign. Over the last few weeks, Republicans have aired three questionable ads against Ford, the latest so blatant that Corker condemned it and asked WHIN radio in Gallatin, Tennessee, to stop airing it. In the first 24 seconds, the one-minute ad attacking Ford and his father, and paid for by Tennesseans for Truth, uses the word "black" six times and accuses Ford of favoring African-American issues above others. "His daddy handed him his seat in Congress and his seat in the Congressional Black Caucus, an all-black group of congressmen who represent the interests of black people above all others," the narrator says. Station manager Jack Williams says he pulled the spot hours before Corker's staff contacted him and that it aired just once.

While the ad was not sanctioned by the Republican Party, it came on the heels of two that were: an RNC television commercial that concludes with a backlit figure of Ford striding into a dark hallway and towards the screen in a manner reminiscent of Willie Horton, and a fund-raising mailer designed by the state Republican Party bearing black-and-white photos of Ford that make him look much darker-skinned than he is and uses phrases including "purports," "pretends," and "passes himself off as" - all terms once used for light-skinned blacks who pretended to be white.

State Republican party Chairman Bob Davis has called the allegations of racism ludicrous, but whether the photos were intentionally darkened does not matter, says Robert Parham, executive director of the Baptist Center for Ethics. "The only plausible reason to use such a picture is to play the race card - in an effort to frighten and fire up white voters in a key senatorial race," Parham wrote in an editorial on the Center's website. "Whether they acted with malice or moral callousness doesn't really matter, the end result is race as a wedge issue."

Time magazine also covered the PNAS study, but also added an interesting twist:

While other factors may not have had much influence, when it came to biracial candidates at least, political views were strongly correlated with bias. In one study, participants were also shown photographs of John McCain. No bias toward lighter or darker skin tone in images of the former presidential candidate was evident, regardless of participants' politics. Yet when examining images of candidates of mixed ethnic backgrounds, bias was plain. "Across the three studies reported here," the researchers write, "we found that partisans not only 'darken' those with whom they disagree, but also 'lighten' those with whom they agree." The findings suggest that race bias is very much alive and well in the U.S., and more insidious than we might like to believe. The researchers highlight several examples in which race, or more specifically "blackness" was emphasized to a public figure's detriment-the scandal over whether the Hillary Clinton campaign had deliberately darkened Obama's complexion in a video ad or, alas, when TIME ran a deliberately darkened photograph of O.J. Simpson on the cover following his arrest in 1994.

Yet while such examples speak to the ongoing problem of racial bias-and how it can be exploited in politics or in the media-the study's authors suggest that these findings, (and perhaps Sammy Sosa's recent effort to lighten his skin), point to a more insidious problem. "Our results suggest that voters themselves may alter how they see a racially ambiguous candidate, depending on their own level of support and their corresponding desire to see the candidate favorably."

The second issue of darkening was done by Time magazine, during the OJ Simpson trial. In Time's review of the PNAS study, it refesr to its own misstep of darkening Simpson's features during the height of the trial. Because of its choice, Time magazine issued a major apology to their readers. The Museum of Hoaxes explains:

Time magazine decided to use this mug shot on its June 27th cover (top), but first they asked photo-illustrator Matt Mahurin to artistically interpret it. Mahurin darkened the photo and reduced the size of the prisoner ID number. Time managing editor James Gaines offered this description of the resulting cover:

The harshness of the mug shot — the merciless bright light, the stubble on Simpson's face, the cold specificity of the picture — had been subtly smoothed and shaped into an icon of tragedy. The expression on his face was not merely blank now; it was bottomless.

However, many people responded to the cover far less charitably. Critics charged Time with racism, claiming that by darkening Simpson's features the magazine had emphasized his skin color and gave him a more "menacing" appearance. Benjamin Chavis of the N.A.A.C.P. argued that the cover made Simpson seem like "some kind of animal." Journalists suggested that, since the mug shot was a news photo, it should never have been altered at all.

Unfortunately for Time, its rival Newsweek ran the same mug shot on its cover (bottom) that week, without altering it. The two covers appeared side-by-side on newsstands, making Time's decision to darken the photo far more visible. Time later issued an apology to its readers.

From a purely visual standpoint, darkening someone's features is a standard way to make them look more sinister. If we are presented with a face cloaked in shadow, it's an indication that this is not a person we should trust. However, because race is entangled into so many parts of American life, and so much of American racial history has been based in denigrating those who are dark, and exalting those who are light, actions like darkening a person can be read in many different ways.

Political Partisanship Influences Perception Of Biracial Candidates' Skin Tone [PNAS]
The Politics Of Skin Color[Politico]

Related: Campaign '06: The G.O.P. Gets Nervous In Tennessee [Time]
The Politics Of Perceiving Skin Color [Time]
O.J.‘s Darkened Mug Shot [Museum of Hoaxes]

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The lead author of the study, sociologist Dr. Shyon Baumann, the Independent, "What the research shows is that our aesthetic preferences operate to reflect moral preferences. Within our cultures, we have a set of ideals about how women should look and behave... Lightness and darkness have particular meanings attached to them and we subconsciously relate those moral preferences to women."

In the advertisements analyzed by Baumann and his team, the skin of white women was 15.2% lighter than the skin of white men, and the skin of black women was 11.1% lighter than the skin of black men. The other major finding of the study was that ads with darker-skinned women were more likely to show those women in their skivvies or with bare feet. (???) Was Anna Wintour falling prey to subconscious assumptions when she and her editors cast the LeBron/Gisele cover?

How Complexion Sparks Desire [Press Trust Of India]
Men, Women & The Secrets Of Skin Colour [Independent]

Earlier: Michelle Obama Is Not A Lighter Shade Of "Trophy" Wife