Cheated

So I had suspicion that my husband was cheating last year while I was working but had no proof. 2 weeks ago I found a pic on his phone from last year where he was on his knees with the women's legs in the air. The pic didn't show him actually inter her but its all the same to me. When I said something to him he denied it and I simply told him that I could show him proof (I made copies) and he was outraged. He said that i just need to let things go, its over.