If you don't want to have a boudoir photo session, you could slip a little less risque of a photo in his pocket. Maybe a picture of you making a funny face, or a picture from the early days of your relationship. If photos aren't your thing at all, write him a cute, funny or even sexy note and slip that in his pocket.
It's sort of like the little notes your mom used to leave you in your lunchbox. Just a nice way of touching base on what promises to be a very busy day.
Although this is meant to be private, remember that private things have a way of becoming public, especially when alcohol is involved! Before you slip that note or picture into his pocket, imagine your slightly tipsy best man accidentally grabbing the groom's suit jacket instead of his own. Don't slip in anything that would mortify you if it became public!