OP, your title suggests there are 'insecurities' about pictures - whose are we talking about? If you mean yours I think the best thing is to talk to your partner about them, though she may want to know what on earth you were doing finding those pictures. If you want to have that chat, I think it's important that you phrase it like that - that you're a bit insecure and you'd like some patience/ understanding/ reassurance.
If you mean her insecurities, then you might want to have a think about why you asked an intimate partner whom you believed was insecure about her body image to send you risqué pictures of herself. That isn't something most people do in their sex lives, and whilst there's absolutely nothing wrong with it per se, you shouldn't be assuming she'll do it or provide you with a reason. Again, did you phrase this as 'this is a bit of a thing I've got, and I'd love it if you sent me...', or did you imply that she needed to explain why she didn't want to?
I'm not trying to deduce anything about your girlfriend from afar, but you do see that someone who feels anxious about their body image and sexual confidence may not be delighted at the idea of sending intimate photos to a partner? There doesn't need to be a trauma and a ghastly story behind all this (tho there could be) for her not to want to do it.
These photos you've found (in a way which may not increase her tendency to trust you with intimate images) have nothing to do with your relationship, I'm afraid. Nothing at all. You don't have a right to an explanation. Maybe she tried it and didn't like it, maybe he was keen on it and she thought it was worth the attempt, maybe it was an attempt to make herself feel more secure. (Maybe he was even the kind of guy who thinks that women who are unsure about their body images just need to be shown they can compete with the porn he watches via a naked photo shoot - that's a kind of guy I'm certain you don't want to find yourself being.)
There's an attitude to sex that men often find themselves slipping into: that sex is a valuable thing which women have and give to men. It's an old an destructive way to think about sex, but it persists throughout our society. Women as sex gatekeeper, or sexual commodity. If she gave it to him, why doesn't she give the same to me, goes the thought pattern. That's not fair, etc. There's just a hint of that in your post - the idea that you have a right to the sexual activities she indulges in, and that anything she's done in the past must be on the menu with you, or you need a reason. That's a bad road to go down, so avoid it. Women aren't sexual service providers, and they don't need any reason to do one thing with one guy but not with another.
I'm glad you've found the board a useful place to pick up good advice about relationships - I suspect your understandable (but not really justifiable) hurt feelings In this situation might be clouding your judgement, so do listen to the advice you're getting. Not necessarily from me, but from others round here!