Just freeze and save the entire contents of memory, paint program included, using an equivalent of an Action Replay (provided one existed for the Amiga at that time).
Datel didn't come out with an Action Replay cartridge for the Amiga until the A500 (ie, a few years after this). I still have my ARII- a truly amazing piece of hardware. In addition to letting you freeze a program and dump memory to disk, the cartridge had a ripper option that let you graphically pan through memory to look for images to save (another option searched for music trackers).
