I have a Fruit Jam and several other board (including the Olimex). It's great fun and I'm enjoying programming it. The biggest delight is that it's actually possible to reason about performance, extremely difficult on larger computers because of all the complexity.

That said, Photoshop doesn't feel like the sweet spot of what to run on this hardware. There's only 520kB of fast static RAM, and that's not enough to run a framebuffer with enough bit depth to hold a photo reasonably. I have invented a 4bpp compression scheme which opens up possibilities, but it's asymmetrical and in its current form compression is very slow, so it wouldn't be suitable for interactive work.

As a general matter, emulation makes a ton of sense. You can emulate all kinds of computers up to the late 80s, including video subsystems. Of course Mac and PC don't really do anything interesting like tiles and sprites, they're pretty much just dumb framebuffers.