Well I can't speak for PSP because I don't use it but I imagine it has something similar to "save for web", it's a setting in Photoshop that effectively reduces the file size of the image, for instance a 800K jpg can be reduced right down to 100K or so without losing much in the process. At that low though it can be noticeable so I usually drop it just enough to get under the 500K limit here!
I'm pretty sure PSP has something similar because I seem to recall someone mentioning it before!

Injustice will be avenged.
Cofiwch Dryweryn.