People will tattoos will surely agree with me, but in case let me give an example:
I DID THIS is not the same as I TRACED THE LINES FROM A PIECE OF PAPER ON AN ARM.
In brief, tattooist are NOT good at drawing anything themselves, they NEED a guide.
Also, I have a tattoo that covers another tattoo I did when I was too young: the tattooist couldn't get the size of the image needed to coved my old tattoo (it was just a cross .... as simple cross, like this +) ... well, he looked so troubled that i was seriously worried he would messed up my arm ... so I suggested to trace the draw my on the piece of paper, then work out the picture (cross( to cover it).
He literally fell from the sky. What i said was such a revelation for him! I could see in his eyes he was like blind man who regain the sight!
Also, tattooists wanna do the tattoo THEY like, not what you like .... they always try to steer your preferences to suit their "artistic mood".
PS: I shouldn't say this! In case my reply may sound arrogant, rude, nasty or too ranting, please, do not think so ... I just had 5 minutes to spare and I used it top reply your question in full.
Explaining something sometimes gets confusing by the length and the choice of words used, and since English is not my first language, I am never too sure that my message gets thought as intended.