Agreed. While you could write something in Perl to solve this problem, there's no point in re-inventing the wheel. if you don't have it on your system it's available from rsync.samba.org. On the other hand, it sounds like you may not be creating a mirror of the entire repository of pictures, just making a local mirror of a picture set whose members change nightly. If the latter is the case, there's a few fun ways to approach this:

• If you have shell access to your remote site, perhaps you could write a script that created a directory of symbolic links to the files you want to download. You could then run the script nightly and then use rsync to create a local mirror of the remote directory.
• If you don't have shell access to the remote site, then you could make the hash like you describe above.

Good luck with it. :)