Hi,
The trouble with a download log, is it could only capture downloads via the zip/download buttons. If a user just dragged the picture onto their desktop or into a folder on their machine, it wouldn't catch that.
But... It is fairly easy to do. Just look through the PHP code (sorry, I forget which files off-hand) to find the bits which activate the download, and add a line in their to either write the details to a text file or to a sql table.
Sorry I can't write the code at the moment, or be more specific, but I don't have access to our servers from here, so cannot look at any of the 4images files.
Thanks,
Alan.