I've just stumbled accross a small piece of code, written one year and a half ago, that blends two 512x512 RGBA 5551 images. It was originally written for a (good!) GIS, so the piece of code blends roads with rivers (and displays the result in a GdkPixbuf). The only thing interesting is that it uses some MMX, SSE2 and
rdtscinstructions. You can have a look at the code in its git repository.