I think she's totally out of order. She's entitled to her feelings, sure, but she should also realise those feelings are (a) unfair and (b) somewhat irrational. And she certainly shouldn't be off with you about it after two weeks. Bearing a grudge isn't going to solve the problem, for you or for her.
While you can certainly make compromises to accomodate her on this -- not looking at pictures to masturbate if it's really going to upset her that much -- she should be doing some compromsing too, or at least agreeing to examine her feelings on this and trying to overcome her anger over what is, in reality, a harmless, meaningless act.
Talking to her about what upset her most about the incident might help you both to get to the bottom of things. Is is that she thinks you don't fancy her as much as the woman in the picture? Maybe she thinks you want her to look like that? Maybe she feels insecure about her body and seeing you masturbate over another woman compounded that? It's unlikely to be as simple as "I don't want you to do that", and if you can pinpoint her exact fear, you'll be a long way towards resolving the incident.