Jonesy - 2:22 am on May 25, 2010 (gmt 0)
"The first 256/512 bytes angle" merely counts on either the meta data or the actual image data in the JPEG being different for different images -- even for 'identical' images from a fixed position camera. There'll be a different timestamp, or different image content(s), or even different lighting resulting in different JPEG compression.
I dunno if it would work in all situations. However, I tested it for a few instances (all fixed position web cams) where I wanted to employ it, and IWFM. IIRC, I started out at 1024 bytes and pruned it back to just 256 bytes with no problems missing a new image.
Back to the OP, if your images can be fetched by HTML services, perhaps the "HEAD" command might do The Trick.