What the heck is wrong with my aunt??!!?

Maybe she is a prude and she feels offended by pictures of young sexy people showing some skin and she thought you where reading something like Playboy, because in her head that is the only kind of magazine that would put a young sexy almost naked woman in the cover. Personally I don't think that what you did was wrong. I just think she misinterpreted the whole thing. And that she exaggerated a bit.