You could use "interlaced" or progressive images. (I don't mean scanlines like old TV signals.) GIF, PNG, and JPG use the same idea: it sends stores the pixels out of order, and progressively fills in the image all over instead of from the top down.
Interlaced GIFs use a three-pass system. Interlaced PNGs use a 7-pass algorithm. Progressive JPGs actually decompose the frequencies and send the "most important" ones first. http://www.libpng.org/pub/png/book/chapter08.html#png.ch08.d...
