None of the above ...
Look at it logically. He knows that you look at his text messages. If he were having an affair behind your back, he would either hide his phone or at least tell his mistress not to text him with anything incriminating because you might see it. So the picture was most likely intended for YOU, not for him.
Why would she do it? There are a few possibilities ...
1) She is obsessed with him and is hoping to split you up.
2) They have an occasional (semi-annual) relationship, and she is trying to "out" him to you.
3) She accidentally sent the picture to the wrong contact.
4) There is some unknown context for the picture (did she have a new "butt job"?)
5) It was an ill-conceived joke and she didn't know you might see it.
Here's what you should do ...
Tell your BF about the picture, apologize for having deleted it without telling him, and CALMLY ask him if he knows why she would have sent it. Then shut up throughout his explanation but keep a close eye on his reaction. You should have your answer.