Have you considered adding the ability to use dithering on this site? I made a similar tool a while back (to convert an image to an arbitrary color palette), and one of the main things I learned was how powerful dithering can be.

For example, here are two images I created using my old tool to convert an image to the C64 color palette. Unlike the website, my tool isn't currently capable of doing the double-wide horizontal pixels, but it still illustrates the effect I think:

Without dithering: http://imgur.com/nQL0y6i With dithering: http://imgur.com/jfwUgnC

If you're interested, Wikipedia has a great example (with code) of the Floyd-Steinberg dithering algorithm, for example: https://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dither..., which I imagine wouldn't be too hard to integrate into your site.