Patagonia brings up some very good questions, particularly about whether or not you talk openly about sex with your girlfriend. If you don't, and if there are things you disagree on that aren't communicated because of that, that causes even more problems within the relationship. Maybe you should sit down with your girlfriend and have a talk about everything you guys want out of the sexual part of your relationship. This could be a good way to lead into talking to her about the photos and asking her exactly why she dislikes it when you look at them. A better understanding of things can certainly help. If you don't think that what you're doing is wrong (and I certainly don't, if it's just pictures), tell her so and explain why. Maybe she could better understand you, too. If you two still disagree on the topic, you two will have to work out a compromise.

At any rate, I sincerely hope you two can work it out.