The situation you've described doesn't make sense on its face: One employee is free to post "gay pride stuff" on his locker but you are singled out because of a picture of gay angels. Did you ask this supervisor to explain why he or she thinks your postcard might be offensive while gay pride illustrations are not? I would not assume anti-gay bias is at the root of his or her request, given how the other openly gay employee has been treated. Angels have a religious overtone for many people. Perhaps this supervisor is reacting on the basis of religious beliefs, or maybe another employee complained on that (or some other) basis. But, still, it does seem unfair. If I were in your position, I'd ask the supervisor why he or she thinks my postcard might offend.
If the supervisor gives you a more concrete reason to conclude that you're being singled out because of anti-gay bias, you need to know your rights and decide how hard you want to fight for the right to display your postcard. If you work in a state or locality with an anti-discrimination law covering sexual orientation, you have legal protection in the workplace. Moreover, most of the major U.S. airlines have policies prohibiting discrimination based on sexual orientation.
As for the displays of "almost naked women," if these offend you, you have the right to express that complaint to your superiors. Such a display might be seen as creating a hostile workplace, and the company would be obliged to address your discomfort. Please keep in mind, however, that I am not a lawyer, and nothing I've said here should be construed as legal advice.