If this is a one-time operation (for each photo), rather than resizing on the fly (each time a page is requested), you shouldn't run into too much trouble. Just to be nice to others, and avoid CPU load problems, I'd do one or two photos and then take a break for a minute. You could probably do more than that at once, but with no way to monitor your CPU usage (except by arrangement with support to have them tell you on a regular basis), why take the risk? As it's a one-time job, I'd just take the 4 days or so to do two photos per minute. To avoid having to be connected for that length of time (and hitting some time limit), you might want to run your script as a cron job -- say, fire it off every 2 minutes to do 4 photos, keeping track of where you are. The script might remove photos from one directory as they're processed and written to another directory, and just do the next photo in the source directory until it's empty.
If you don't want to do it on your PC because of all the labor involved, some image manipulation programs have scripting languages that may let you fire it off on your PC and it runs overnight. GIMP has such a language, but I've never used it. I wouldn't be surprised if Photoshop can do it too. You might be able to cobble up some sort of batch script (e.g., in Perl) to run your command-line batch conversion/scaling program non-stop against each photo file. Of course, you then have five times as many files to upload as the other way (but any good FTP client should be able to do more than one file at a time).