I can tell you as a 32 year old man that has been married for almost 12 years that your boyfriend is normal and can be trusted based on what you've said. Everyone fantasizes, and more often than not the fantasy is either with someone you won't ever be with, or about something you'll never do, and sometimes both at the same time. Hell, sometimes 20 people at the same time!
Point is, doing something nasty with someone else in your head is a release. It relieves stress, it relieves sexual tension and it feels good and makes the rest of the day better. Once it's over, oftentimes the acute attraction is diminished and you move on. It's not a fixation, but rather a release of the fixation that resides at the back of the mind. So that is why it is normal and healthy.
On your side of the issue, you focus on the one person and expect the same in return. That is not fair for either of you. You
will be let down if you continue that thought process. There are 2 things (among many) that I think you should consider: 1) Every person fantasizes differently, so what works for you doesn't necessarily work for him. 2) You seem young, and you haven't learned yet that digging into male fantasies is a mistake if you are the jealous sort. Never ask a question for which you don't want the true answer. He can either lie to your face or he can blow your mind (in a bad way), so by touching on the subject of masturbation fodder, you put him in a spot for which he cannot win.
This also brings up another topic. If you are digging around in his files, stop. You will either have to trust him or stop trusting him, at which point the relationship is over. I can guarantee you that if my wife dug around in my files, she would probably be somewhat angered by the number of fantastic butts and hot chicks I have scattered around. I don't dig in her files so I wouldn't know if she has a secret stash or not, but I suspect not, now. She did have one once, but I ran across that by accident. I didn't bring it up and I haven't seen one since, but it doesn't matter. She's not doing them so if she wants to get off to some pic, more power to her
If you and your boyfriend are serious, try to trust him more and move on (and quit probing). Otherwise, move on to someone you feel more comfortable trusting (but still quit probing the male mind, unless it turns you on).