I have tried to save sprites as a .jpeg and .png but when I try to open in GIMP it says the file is an unknown file type. Does anyone know how to save a sprite as an image I can work on in gimp?

I right click on a sprite and then press save to local file.
Choose where you want to save it and give it a name.
After the name put a dot and then png like this: .png
Then you can open it in GIMP.

If that does not work, GIMP may not be working.
Try opening another file with a .png extension. If the file does not open then GIMP is not working properly,
or maybe it is an old version if GIMP that cannot open .png extensions.

Hope this is useful.

If you cannot find any other way, screenshot the sprite you want to use in GIMP and then open it from there!