The devil’s in the details, they say.
To make it impossible to tell that it is fake would likely require an extraordinary, impractical, and maybe even impossible level of planning and effort.
At the end of the day, the photo you craft is fake… so it would simply be a matter of time for someone to crack it.
Now, if you’re looking to make an attempt that would pass casual inspection, like for a family Christmas card or something… well, that’s actually not too hard.
But if you’re trying to beat the pros who have the forensic tools and know how to use them (even if you’re one yourself), I’d look for something else to try.
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I’ve done the Christmas card bit… little kids and pets can be so persnickety trying to get everyone looking at the camera and smiling.
No one could tell that I’d replaced heads, filled-in missing arms, etc… even after pointing it out.
But I’m sure a pro couldn’t pointed out all the edits in under an hour, with the right tools.