Multisession DVD-R
 

I only have multisession DVD-Rs left. Can I burn an iso image onto them? (and have it work) If so, how?

Nope. An image file requires a complete disc; that's the whole point of having an iso file, it's an image of the disc. The disc is completely mastered using the iso image as "instructions" and contents. There might be a way to burn the data onto the disc but I'm not sure if it can be made bootable and work like it should, so you better just get a blank disc. Preferrably a DVD+/-RW so that you can overwrite it when needed.

so what good are they?
 

If multisession discs cant hold an image, how do they work? I know Puppy Linux can use them to save files back to the disc...but I have not the slightest idea how.

can you use RW discs?

Yes RW is nothing but another empty disc, but like he said, here again you gotta give the entire disk to the image file.