Hi, brazil, and welcome to the Eastwood Web Board.
I've moved your post here to Questions & Answers, in the hope that someone might be able to answer your question. I've just looked through the Paramount presskit for Escape from Alcatraz, but there are no behind-the-scenes shots there.
We do have a number of serious Eastwood collectors who post here, so maybe one of them will have something for you.
I know I've seen a shot of Clint (out of character) exploring the prison, but I can't remember where, probably in one of the numerous picture books I have. It's not in Don Siegel's memoir A Siegel Film ... there is one behind-the-scenes shot, but it's just Clint and Don Siegel standing in a cell block, probably not too interesting for your purpose.