here we go: advice from JM
1st) how old is she?
2nd) no you're not wrong, granted its not like you have GIGs and GIGs of porn on your computer, there was a single picture blah blah blah.
3rd) she is obviously rather insecure if she thinks that you're either A comparing that girl to her or B wanting something else when your CLEARLY with her.
4th) Giving the ring back shows that she doesn't truly take this whole thing seriously, because (assuming is an engagement ring) thats a BIG commitment to just "give back". However, not knowing this girls past she could have a HUGE thing against porn, which some girls do. My girl gets mad when she finds a whole file devoted to asian girls

but still we talk it out and she knows that guys are attracted to girls (or SHOULD be) but that she is the one for me and that there is nothing she has to fear. girls are like puppies. they follow you and are looking for constant reassurance that you love them, need them, and just all around care about then and are NOT looking to replace them.
FIF) girls are spiteful little bitches, they do the same thing guys do only they make sure to "hide it in plain sight" and you don't know if its really that guy or just a friend (guy or girl) that she is doing certain actions (deleting in front of you) to make it look like someone is interested in them so you better be nice. Girls assume this makes them more attractive and desired when they simply don't realize it pisses us off more. anyways, don't jump to conclusions cause it will cloud your clarity and just all around complicate things.
best of luck buddy. if you sit down and talk to her and just listen to what she has to say and set some rules or something, she should be a big enough girl to explain clearly what the problem was and how that made her feel.
hope i helped.

-jm