You don't even need to display them, just tell his girlfriend that you're playing a joke on him and all she has to say is that "Your mom showed me your baby photos of you on a blanket and you were so cute!" Been there, did that except that in our case it was a photo of cousins James (mine) and Megan around the age of 1 1/2 or 2 in the tub together, both soaking wet.
Come to think of it, I should let Uncle Dave know he has some ammo to use on Megan too.