Does not seem to work
You are right,
Picture.Name is only the file name, not a complete path, checked myself now to be sure.
Probably you have also seen
this on StackOverflow already.
Looks all mixed-up and pretty hopeless to me. If you, I and a couple of other people have not found a way so far, there probably isn't any to do what you want - the WP7 API really is still imature and full of holes waiting to be filled.
I personally try to take such things as an intellectual challenge: Given the many things that are not possible (in your case selecting multiple pictures together), what can I manage to achieve despite these limitations?