thanks a lot for this amazing feature! I already gave it a try, and it offers a fantastic insight into the origin of a jpeg. unfortunately, some digest collisions occur between pictures from cameras and applications, e.g. definitely unmodified photos from my olympus e410 are reported as "Apple Quicktime Quality 845". furthermore the camera seems to vary the compression, because pictures with identical settings yield different compression signatures (some of them unknown).
So while my hope to uniquely identify camera models by compression digest was a bit disappointed, i made a quite interesting discovery: Photosharing Services such as Flickr, Picasa or Facebook all use JPEG Group Library to generate their preview images, but different Quality settings are applied: Flickr seems to use 90 for thumbnails and 96 for preview images, Picasa and Facebook use 85. This setting is global and should apply for all preview images. So one could use image dimensions in conjunction with quality settings to distinguish between downloaded pictures from Flickr/Picasa or Facebook or at least narrow down the possible source to some online services.- Franz