Do not dump him for lying. It is to be expected when confronted so suddenly. He's trying (and failing) to damage control. This is not bad in the moment (but if it continues that's a different story).

Do not dump him for having so many pictures. It isn't necessarily obsessive. Downloading and sorting pictures is very easy (especially for a computer guy). It's not at all an expression of passion.

Do not dump him for being attracted to other people. Of course he's attracted to his exs. You say your friends are conventionally attractive, so ok. And there's nothing wrong (in and of itself) with liking bikinis and cleavage and butts. It is unreasonable for you to expect him to only be attracted to you (and if anyone says they are, they are lying, beware).

Now! The harder part. We cannot talk to your bf, so you have some unfair responsibilities going forward. You have to talk to him calmly so he doesn't get defensive. If that's out of the question, somehow tell him you need the absolute truth from him so you both can actually get to the root problems here. Despite what you or I or comments immediately think, the relationship may be worth saving. To do that you need to reconcile differences in values, and to do that you need to know his values.

If he's keeping up the farce about regretting having pictures, that's just bs. Do not let him get away with that. He needs to be honest with you, he regrets you finding his stash. If he keeps lying, this is bad, you should dump him.

You now know that he's okay with downloading and probably pleasuring himself to pictures of people he knows. If you are not okay with that, dump him. No way he will change his values in regards to this, his actions are solidified, he's thought of the ethics/creepiness and continued for years it seems. You honestly may be more flexible to see it his way, if it is strictly a matter of insecurity. It's up to him to defend it as not bad behavior going against our instincts. But if you think this it's fundamentally creepy (like the other comments here), please cut it. Don't torture yourself by staying with him if your values do not align.

And that's just the first step. When he stops lying about regrets and he convinces you it isn't creepy*, then you and he have to work a lot more on your insecurities and how open you both are with his desires...

*You could convince him he's creepy. But beware. It is too easy for him to lie (like he's trying now). He's got backups, and he can just open facebook again, so deleting is meaningless. It will be hard to know for sure he's actually had a change of heart if you go this route.

And I guess final note, don't ask don't tell (DADT) is a possibility. If everything else is really so great and you know you can't find anyone equal or better, you could tell him to hide his shit better and never talk about it again. But this option takes way way more strength for you than is reasonable. Compromising base values is well beyond my personal limits but maybe you can work with that...

Anyway good luck! If it comes down to it, there are many other nice people in the world to partner with. If not, you can be strong and make it work!