Unfortunately, most guys love porn. You should sit down with him and tell him how it makes you feel. Ask him how old the picture is, and if he has it to masturbate to or why its there.
It is okay in my opinion for guys to watch porn, but it is not okay if the porn interferes with your sexual relationship. If that is taking a front seat to your sexual needs, then you really need to talk to him about it.
Maybe you can try incorporating porn into your sex life together? Could you look at pictures or watch a video with him and touch each other while watching it?
You have every right to be upset if he is choosing pictures over you, but since it is only one picture I think you should ask him and let him explain first to see what he says about it. Good luck!