Good one Mr. Floppy!
For anyone stumbling across this looking for something similar for Sauerbraten: Pretty Screenshot

Two things though:
1) You don't need to wrap all those commands in brackets - that's redundant
2) but this is just personal taste I guess: I would keep the file seperate and just call it ("exec config/clean_screenshot_script.cfg") from autoexec .. but, of course, both methods work just the same :-)