I think the sooner you show them to her the better. Does she know that you communicate with her birthmom? If not, she may not even think that you could possibly have pictures. If your daughter already knows she was adopted and you have been open with her in answering her questions so far - I'm sure she is not only going be able to handle seeing pictures of her birthmom, but she will love it too!
Perhaps in conversation one day you may want to first ask her if she would like to see a picture of her birthmom. If she says yes, then you have your answer. If she says no, then you can tell her that when she's ready to let you know.
We are in an open adoption and DS sees his birthmom occasionally, but I have a picture of her (with his brother and sister) in his "Who Loves Baby?" photo album so he can "see" her every day.
Please keep us posted - I'd like to hear how it goes.
