Gameboy Color picture, the constraints being:

• 160x144 resolution
• no more than one 4-colour palette per 8x8 block
• only 16 4-colour palette for the whole image (so that's theoretically 64 colours maximum but generally you end up using a lot less than that as you use most of the palettes with different combinations of a few colours)

This was made for a GBC demo.

I used this picture of Aishwarya Rai as a reference.  (You might need to refresh the page to see the image...)