Digital photographs are full of information that we don't usually see, and JPEGsnoop can extract this for us.
Much of this information is not interesting, but JPEGsnoop uses it to see whether a photo has been altered at all. Want to know if a picture has been retouched? This will tell you. Want to know what program was used to edit a picture? You can find that out too. All you have to do is click the open file button, choose the picture, and moments later you'll have a very lengthy analysis.
Scroll to the bottom to read the assessment, which will be graded from Class 1 (definitely processed/edited) to class 4 (uncertain if edited or original). Apparently it's difficult to be certain if a photo is "original", but easy to ascertain if it has been tampered with.
Of course, most digital pictures have been compressed at some point, and editing in Photoshop has more or less become a norm, but it's still interesting to look at. The image reports are too technical, and not presented in a very user friendly way, but the conclusions are clear enough.
JPEGsnoop is a powerful analyzer, and if you're interested in the origins of a digital photo it will give you a wealth of information.