Is he communicating with you?
Who broke up with who?
WHy did you split?
How old are the two of you?
It could be that he's holding onto the memory card as a way of punishing you for what happened. Although he wants his keys he's willing to sacrifice them at the moment to make you feel bad. It could also be that he's using them as a way to keep a link to you, to hold on through good and bad, regardless of how you feel. The simplest reason might be that he, like many other people is a procrastinator and hasn't gotten an envelope, stamp or made it to the post office.
Try reaching out to him and changing tactics, be a little nicer, even though you may not want to be. Let him know that you want to get your pictures and that they are important to you and how much you appreciate him holding on to them for you. Ask if he needs you to send him an envelope and stamp (this way you might be able to get the address).
I hope this helps, let me know if you want to talk more
Why do you say you shouldn't have, at the very least you found out what you needed to know and if he did in fact mail it,then you can send him his keys and move on with your life.