Is that something that works with any browser / server, or was that somebody's "proprietary" extension?
Bodidily says: AFAIK, this happens soley because of the chosen file type. I think if you have a large .gif image, it will load by sharpening, whereas a .jpg will seem to 'scroll' down (like a butt ugly PowerPoint transition!).
Jerry Muelver says: You may be viewing interleaved or interlaced GIF (interleaving or interlacing) or progressive JPEG. See http://www.uga.edu/ucns/dms/webclass/image-formats.html