The birthmothers for my two children have LOVED getting letters and pictures. Their favorites are the everyday stuff. What makes your baby smile? What makes him/her cry? They want to hear how much you love your child. They want to hear from the dad, too, since that is a big reason they placed. And don't forget to tell teh birthmom how much she herself means to you, how you honor and love her, and know how much she loves her baby.
As for the pictures, send a lot. And not just posed, cutesy pictures (although, at three months, that's about all there are!)
But a little later, sent the covered in oatmeal ones, or the one with him wearing underwear on his head.
Many birthmoms LOVE getting handprints or footprints.
Good luck! I sent letters every month for the first year, to each birthmom. Although it was so har sometimes (didn't feel like it, was EXHAUSTED, had no time, etc.) I now have a journal detailing my children's first year - all their adorable and funny things, and how much we love them. Otherwise it would have all disappeared into a big blur. I love to go back and read them, and my kids now love them too.