I know this is old, but I've figured it out. Ironically enough, the information is all there on the AGI wiki. To use SAVEPIC you need to utilize the included template.pcx file. There's a white outlined 160x168 square in the center of the image and that's where your picture needs to go. Strange they chose to do it that way, but anyway.
I haven't tried making priority lines or anything, but I was wondering if the AGIPAL tool would help change AGI256 palettes. On the one hand it's pretty neat that AGI can have 256 colours, but it's not really usable if you're stuck to a default palette. I was considering "upgrading" SQ1 images in Deluxe Paint much like how Indy3, Loom, and Monkey Island 1 were. But you can't really take advantage of all the possibilities with such a limited palette.
It's probably not possible, but does anyone still have the AGIPAL tools? They can alter the 16-colour AGI palette. It's listed on the AGI wiki but the download is not included. I don't seem to have it in my backups either.
EDIT: Never mind, I've found them and uploaded them to the wiki.