Maybe you knew about metadata (that is, data on data) with digital pictures but I did not. Not until I read Beware of hidden digital camera metadata from Canadian Privacy Law Blog and Authenticating Digital Photographs as Evidence: A Practice Approach Using JPEG Metadata from Law Practice Today. This paragraph from Canadian Privacy Law Blog sets up the problem nicely:
There is no better illustration of the problem than the website created by Tonu Samuel. His site pulls images off the 'net then shows the original thumbnail and the modified image. One image generated by Samuel's site is a very vivid demonstration of why this is an issue: Hidden EXIF thumbnail security problem (may not be safe for work - it shows a young woman in a bikini whose face was obscured but is clearly identifiable in the thumbnail).
In short: Be very careful when you distribute modified digital images.
So what? Think about this scenario: digital photos come into evidence and then the metadata shows the photo's alteration. I will bet the party putting that photo into evidence gets chewed up and spat out.