The close-up image shows a welt above each of the woman's eyebrows, marks on her cheek and around her lips, and general swelling. TMZ did not say how it obtained the photo, when it was taken or by whom.
Hours after the photo was posted, the Los Angeles Police Department launched an internal investigation and asked for the public's help in finding the person who leaked it. A police news release said the image "has the appearance" of one taken during a domestic violence investigation, and classified its release as a case of serious misconduct that could result in termination.
Brown was arrested Feb. 8 and booked on suspicion of making felony criminal threats to a woman authorities have not publicly identified. A person familiar with the situation, who was not authorized to speak publicly on the matter and requested anonymity, confirmed that the woman was Rihanna, whose real name is Robyn Fenty.
The person said some of the injuries shown in the photo - specifically the marks on the woman's forehead - were consistent with the injuries Rihanna sustained.
An Associated Press photo of Rihanna, taken before the party on the night of the incident, shows her with a tattoo of Roman numerals on her left shoulder; the TMZ photo shows the same numerals on the same shoulder. TMZ had previously used its site to debunk a manipulated image that depicted Rihanna with a black eye.
No public statement has been issued on behalf of Rihanna, who turns 21 on Friday. Email and telephone messages left late Thursday with representatives for both Brown and Rihanna were not immediately returned.
Brown released a statement on Sunday, which read in part, "Words cannot begin to express how sorry and saddened I am over what transpired." The statement, which did not mention Rihanna, said Brown was consulting with loved ones, his mother and pastor.
Brown's arrest came roughly 18 hours after he and Rihanna left a pre-Grammy party honoring music mogul Clive Davis. According to police, Brown and a woman were involved in an altercation in a car traveling through the upscale Hancock Park neighborhood shortly after midnight. Police have said the woman had visible injuries, and that Brown was gone from the scene when they arrived.