PixelMonkey : Ashley is right. In HTML5, images are stored as matrices, one per color (R,G,B), and Alpha. There's no concept of "indexed paletted image" (i.E. one byte value translating to one entry in the palette).
On that demo, the coder used image from an old Amiga and then DOS code he made. Each image is sent as JSON to the browser, with an index for each pixel translating to an entry in the palette.