As far as I can tell from the gui_draw_splash function in core.c it looks like it's 4bit per pixel (8 colors), 150 pixels wide and probably 32 pixels high (though the file size is not divisible by these values)
later edit: no, it's more like each byte means:
first three bits - color index
last 5 bits - how many pixels to go horizontally, and going vertically each 150 pixels
so something like Run Length Encoding.