So this is a JPEG decoder that runs on an Apple II? Monochrome only, or can it do color?
Monochrome only, and it downscales. Only 1/4th of the pixels' IDCTs are computed, for "speed". It is also hardcoded for 640x480 images, again, for "speed" :-)
It's based on picojpeg, heavily reworked to get rid of about all intermediate buffers, core decoder functions rewritten in assembly.