Script is updated, now also supports PNG and GIF resizing. Tested all filetypes with a 1024*768 document. The source files where roughly around 300 - 580kb, after resizing I had 122kb, 170kb and 194kb.
And support for a .PNG watermark, althoug transparancy isn't support yet I noticed (not sure why), so perhaps using a nice bar with a name/url of your site will do the trick aswell for now
Also added the date (dmY) in front of the image, this should make it easier to clean up the images folder once in a while when running near your upload limit