I so wish I hadnt moved my own thread to ofbt when h was checking me out in October. There was lots of useful stuff on there from re my own experience, which was my h's 5 and a half years of internet sex, it turned out.
I recognise that feeling of suspicion and also of failing to confront effectively, or confronting and having to be prepared to believe him, several times over the whole period, just because he kept denying and having techy explanations for these photos (accidentally downloaded from torrent sites with music, blah blah).
Even after I finally decided not to believe his last explanation, at the time of my thread, he still denied and denied, and in the end it was only me moving out of the bedroom and preparing to divorce which made him confess, and after 3 weeks of Relate sessions, which I was at the time using partly to plan divorce.
The thing is, one doesnt want to believe- it makes us have to reevaluate our own foolishness and naivity, plan a divorce and new life, thrown away all those years, and see our partner/h in a new terrible light- ie the man I believed was different to all of the others is not different at all, and worse than many. It is vile.
You have two options, in my view, or two sensible ones anyway. Either SAY NOTHING at all, so not to raise suspicion- as once you do that, if he lies he wont stop, and is likely to be stronger than you in wearing you down. Evidence will disappear and you wont ever find out, or not for years. You may go round it in cycles like I did, with a year or so till the next slip, and also you will get bored and frustrated looking so much. So option 1- say nothing and check him out as far as possible- look at his cache, search for photos, key words , history (he may be browsing incognito) etc etc- and try to find his password.
Or, tell him right now and dont back down, AT ALL. If you do that, do what I failed to do, and did try eventually, but too late- use the element of surprise in order to make him take you on his hotmail account AT ONCE, and let you look for yourself. Even if it isnt there- you kinow what you saw.
FWIW my h did not tell at Relate but denied- it was only between sessions 3 and 4 that I forced him to admit, over a series of long convesations, where I kept reminding him that I would not be rebuilding our relationship without it.
Sorry, so long, but truly, I feel as though I could take a degree in this subject by now.