Thanks jeromedevine for bringing this thread on top - it would have missed it otherwise.
Thanks kranu for sharing your useful tricks!
A note for people on Linux - just copy kranu's stamp.png file to a directory with your screensavers and run the following command in that directory:
ls *.png *.jpg *.gif | xargs -Ifile convert -coalesce file stamp.png file
Your screensavers should be stamped now (backup the content of the directory before doing that just in case)