As I started to read the OPs post about receiving the text message, I thought you were talking about your Dad and your Sister getting it on! :O
Either way, it's a tough one to call. You could always talk to your Dad about it, just say you found the pics in the recycle bin. If he lies and says it's nothing, then go from there. You can either talk to your mum, or continue with your dad, although letting on you've been hacking into his stuff is not a good road to go down.
Whatever you do, you need to do it properly. Talk to your Dad, find the truth, or talk to your mum about it ("has dad been acting strange" etc). Or leave it alone and forget about it.