Is he cheating on me?

In march 07 my bf had a message on his phone that I checked. I called the girl back of course and she said that they had been seeing each other but didn't have . It took a long time to get over it (like stop checking his messages) but I decided to stay with him even though I didn't believe anything that he said to me. Like she's lying and she is a fat girl that his homeboy gave his number to. Then about 1 month or two from that I seen a pic on his phone that was of a girl that was naked he said his friend had the girl send the picture. So I flipped out of course and I wanted to talk to his friend. Of course his friend would lie for him. Now I check his call log every month. And ever number that I see alot of I get suspicious. I'm going crazy we have been together for 2 1/2 years now and we live together. He doesn't know that I checks his stuff but I can't continue to live like this. I think he's cheating I just want to catch him doing it and don't ume. I need help!