What a great idea! I would love to see a "save AS disk" option that would let you download a D64 disk image containing the rendered graphics in some appropriate format like KoalaPainter or Amica Paint.
There are some good cross-dev C64 graphics editors out there on the PC - I think most of them will let you import pictures, convert them, then save as PRG (etc) executables you can run on a real C64.
PC:
- Timanthes: http://csdb.dk/release/?id=75871
- MultiPaint: http://csdb.dk/release/?id=156376
- ProjectOne: http://csdb.dk/release/?id=86776
- PixCen: http://csdb.dk/release/?id=149752
OSX:
- http://csdb.dk/release/?id=156376