Well, if you are running 6.3(1) you could use "copy tftp://x.x.x.x/pix633.bin flash:image" , where x.x.x.x is the address of your tftp server on a pc preferably on the inside, and pix633.bin is the OS for the pix from cisco. Much like an ios box.
Also you can get it from a http webserver if you wan't that, the syntax is the same just change tftp to http
Jan
Network Systems Engineer
CCNA/CQS/CCSP