Yeah I think that part is a bit iffy, and if you look at the resulting image it does seem to have some fringes around the outside of the cat, indicating that it's showing something related to the curvature of the heightmap, rather than the intended image.

It's probably a good enough approximation to at least generate a recognizeable image. Especially if you use a point source because then each 'window' will reflect all its light in more or less the right direction which will converge on a single point as well. Not sure if they'll all focus on the same plane but it's probably close enough.