I use the random image script for my signature and have noticed that it does take things a bit longer to load.
It seems to display the images as uncompressed bitmaps instead of whatever the original was. I know all of the images I use are compressed JPEG's that I try to keep around 20kb. When the same images are displayed with the script, they are bitmaps around 150-200kb.
I'm on a cable connection so its not a problem for me, but it could be for those 56k'ers out there.