Kramer photographs Gerry with a Polaroid camera which takes pictures sized 3.1 in x 3 in with a border, saying later that he "took a double print". The photograph which he puts up in the lobby appears to be a 7 in x 5 in and has no border. It is not possible to take a double print from a Polaroid which has no negative.
Kramer decides to put all the residents' pictures up in the lobby of he and Jerry's building so everybody knows everyone else. While he hangs Jerry's picture in the lobby you can clearly see his own picture with only "Kramer" written under it. However, a resident ("Steve") comes down, presumably just coming to know Kramer from the picture and shouts "Hey, Cosmo!"
The photo shown in the lobby is not from a Polaroid, but an ordinary paper print. Additionally, Kramer (Michael Richards) took the photo from a distance whereas the photo is a closeup of Jerry's (Jerry Seinfeld) face.