in axe, bitmaps are images that are drawn directly to the lcd, like text usually is, rather than drawn to a screen buffer that is then sent to the lcd all at once. this is useful when you want to have an overlay (say, a menu or something) that doesn't affect the image currently on the buffer. it shouldn't be used for displaying in-game sprites or anything like that, though, because the method of drawing directly to the lcd is much slower than using a buffer (and is, in fact, one of the biggest reasons why Basic games are so slow)
EDIT: runer pointed out that i'm behind the times and the bitmap command no longer draws directly to the screen in recent versions of axe, meaning builder is right and it's just an easy way to draw multi-sized images. if you have a single-small sprite to draw, though, the sprite commands will still be more optimised, and thus faster.
