....and Im still in shock...
when i told her your not taking them.. she said she was and walked out the door.
This is just plain theft. Age is not an excuse for this (and I have relatives that I make excuses for). She KNEW you said she could not have them.
If she is staying at your parents house, then she will have the pictures somewhere at your mother's house. Tomorrow after dinner, ask for them back nicely, pointing out that she did not set a good example for your children and you'd like to see her make it right. In return, promise your aunt to get her copies of the pictures. Tell your mother ahead of time that you plan on asking for them back and hopefully get her support.
Try not to get emotional about it, but speak matter of factly. This is a chance to show your children the proper way to handle this situation. How not to be taken advantage of and how to properly stand up for yourself.
If she refuses, then you need to tell her outright that she is no longer welcome in your home or around your children. She demonstrated disrespect for you and your posessions, committed a crime against you and did not show enough self respect to admit her actions or make them right. These are not the kind of people, family or not, that I allow around my children. Again, you will need to keep your emotions in tact (difficult at best).
Although I know you want those pictures back, the biggest thing to take from all this is a valuable lesson for your children (your aunt and mother too!)
You said that your uncle was there.....did he witness this? Will he support you?