I was recently asked by a solicitor to examine images to determine if they have been digitally manipulated. There are a number of processes I use including Photoshop forensic techniques and Error Level Analysis.
Error level analysis can help you work out if a photo has been digitally altered. Photos in a jpeg file format actually lose quality each time they are re-saved. I can take advantage of this to try and work out if an image has been digitally manipulated.
For instance, if you have a jpeg photo at 90% quality, and resave it, again at 90% quality, you will end up with an image at 90% of 90% (so, 81% quality). If now, you were to copy and paste an element from a different photo into your current photo, you can end up with sections of the image at differing quality levels. This happens because some parts have been resaved three times, whilst some parts may have only been saved once.
If the image has had no alterations to it, all parts of the photo will be at the same quality.
Error level analysis allows you see to see the difference in quality level, represented by brightness. Things which are very bright have been edited most recently, whilst duller parts have been resaved multiple times. This allows you to see not just which parts of the image have been changed, but in what order the changes occurred.
I have used this technique, in conjunction with other forensic tools to support many claims, where the integrity of an image is in dispute.
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