Detectives also believe a dark shape in the photo may be a male passenger in the rear seat.
"We have been exploring ways to enhance the quality of this photo in an effort to learn more about this person," police said in a statement.
Police have not said when the photos were taken or given the location of the ATM, saying only that it is in Chapel Hill.
Carson, 22, of Athens, Ga., was found last week on a street about a mile from campus. She had been shot several times, once in the right temple.
Her SUV was found a few blocks from her home, but police said this weekend they had not yet found her wallet or keys. According to search warrants, police found Carson's cell phone near a shopping center about 1 1/2 miles from the scene of her death. Police also retrieved Carson's laptop and memory cards from her campus office.
Police said Monday they had received a number of tips since releasing the photos on Saturday. The school's Board of Trustees, of which Carson was a member, has offered a $25,000 reward for information leading to an arrest.
In another case involving a female student, Auburn University in Alabama has begun new security measures since 18-year-old Lauren Burk's abduction and shooting death.
Maj. Melvin Owens, Auburn's executive director of security and public safety, said Monday the school has increased the hours of its security shuttles and has implemented an escort service for students leaving the library.
Burk was found about five miles from the university last Tuesday and her car was later found burning in a campus parking lot.
Courtney Lockhart, arrested on capital murder charges, is accused of kidnapping, robbing and attempting to rape Burk. Lockhart, who lived in rural Smiths in Russell County, was arrested in Phenix City on Friday after a chase and was to make a court appearance Monday.
Police have said there was no indication the killings of the two students were related.
Lockhart served with the Army in Iraq from August 2004 to July 2005. At Fort Carson, in 2006, he was convicted on military charges of communicating a threat and use of marijuana as well as assault, said Maj. Nathan Banks, an Army spokesman at the Pentagon. Lockhart was sentenced to seven months' confinement and given a bad conduct discharge.
Banks said the records he has available do not indicate who was assaulted, but the victim would have been in the military since the case was handled by the military rather than civilian police.
