Is it useful? does it make image loading fast?
I recon this is like "Interlacing" which shows a low res version of the image up front then gradually improves the quality. As stated here.
It is seldom used nowadays. If you wan't to load your images more quickly, serve them trough something like the Amazon AWS service.
Picasaweb uses this for jpegs. At first you get the lowres, that get's sharper and sharper until you have the full quality image. It's usefull for large jpegs.
More info can be found here: http://en.wikipedia.org/wiki/JPEG#JPEG_compression