I would like to have some kind of proof that my snake is 100% het, but isn't necessary. I try not to buy animals that I'd sell in the future anyway.
My pastel het clown girl didn't come with a photograph, but I copied and matched up the snake with the breeder site photographs and it matched perfectly.
And while there wasn't any actual paper work, the snake came with a receipt that stated what she was. And I have email correspondences from the breeder that states what she is too.

Either way, the most important thing is to buy from a reputable source.