Cheating problems have escalated...?

About a year or so ago, I was on here about my step dad. I was asking what I should do about an inkling I had about him cheating. He was sending perverted text messages to a family friend, but they eventually stopped. Now, my step father has recently acquired a Blackberry. He invited me to come use it and figure it out, so I could explain it to him. I was getting to know the phone better, when I see a picture of a naked woman who has an uncanny resemblance to the woman he may or may not have cheating with. What should I do? I love my step father with all my heart, but I don't want him to hurt my mom. I'm one of those rare cases you find where a young woman actually appreciates her step dad, and I don't want that to change.