Knicky, I'm so sorry that you are having to go through this too.
RE GEL FWIW....I think you are wise to try to wait for a few days to gather more info. IMPO, a few days will show you that he is either simply viewing this stuff....or that there is more.
I was going to suggest something like this too to GEL but thought I better but-out of her problems. I can imagine it is difficult and emotionally draining to wait and continue monitoring your H w/o saying anything but from my POV, the longer you can monitor his on-line activities and not let him know how you are doing it, the more truth you will have to work with.
In the mean-time how about asking him a couple of hypothetical questions like "why would a guy look at pictures if the real thing was available." You know what he was looking at and could counter his stinking-thinking answers with things you believe you want.
I am not into fixing guys that have the "real thing" but would rather have a picture. I can only guess about a few things why he would.
My W (LD) asked me why I don't have a swimsuit calendar in my shop. I said pictures are nice but give me the real thing anyday and gave her a wink. Maybe she is a littler HD than she makes out to be or is she in some mental state that reeks of "men just want a hot body to look at." I don't know.
feel guilty for "spying" Do you feel guilty about spying or hurt because you trusted him and found out he was doing something that broke your trust.
I think in some way, most good people want someone they can trust alomost 100% and finding out our trust was a false hope, at the time causes us to lose something inside that tells us we should not spy (in this case find out what is true). Maybe it is something like losing a fairytale belief.
Again, sorry about what you discovered. I can say what you and GEL post will help other women and men in their relationships.