Acutally, unless you are a professional specialized in manipulating photos without detection, it is not that difficult to find evidence of digital photo alteration. Most photographers will not be able to carry through digital manipulations without leaving traces.
1. First, "they" are going to want to know what camera you used to take the photo. That is the starting point. Almost all cameras will leave EXIF or other info embedded in the photo. Once you start altering the photo, the metadata invariably becomes corrupted (out of synch with the alterations), leading to certain kinds of contradictions. But forensic analysts have many tools at their disposal, including AI.
2. Any crime nowadays is so hard to get away with undetected - so much of life is getting almost constantly recorded. Every activity seems to leave a trail somewhere. Going through those occasional traffic lights with cameras that identify and record your license plate, ennumerable video cameras seem to be almost everywhere, Microsoft, Adobe, Google, and umpteen other companies spying on you when you are doing digital manipulation on your desktop. You poor fool. You DO NOT STAND a chance.
3. "Someone" can just ask you: "Did you alter the photo?" And you oughta know, if you lie, you may be in serious trouble. So, when it comes down to it, you are probably going to play it safe and say something like: "I'm not sure." And, truthfully, you may very well not be.
4. Your professional reputation? Your personal reputation!
And all that data is recorded a dozen times over - you don't even know where - just that it's there, and there, and there.
Better be safe than sorry.
