It probably happens more than people think, i've never used it and never will.
When i sell something i take about 12 pics and use the one that shows the reptile as is.
I think it's pretty obvious if someone has photo shopped something, most of the time if it looks to good to be true then it probably is.
Also look at the person selling if they have a good name then it's less likely they would risk being found out for something like that.
I can think of one fairly well known breeder thats done it and got found out.