What if she changes her mind after signing the contract and taking the photo?
You also need two witnesses to testify she is of sound mind (and body, I guess) and a notary public. A lawyer, too. For each party.
If it’s an orgy, well, better fire up the printers and copy machines.
“What if she changes her mind after signing the contract and taking the photo?”
Contract law goes way back. Dutifully fulfilling your contract obligation is required.
Now if one or both parties are drunk things could get tricky.