Really?
A contract that states exactly what you are offering, for the $500, and make your service limitations known. In this case, a statement that you are not a pro, that he understands this, and that he agrees not to hold you liable for anything. All in legal speak of course.
In my business, when I did freelance custom jewelery, I covered what happens if I broke a stone, that my limits were to pay for recutting, and to describe all the inherent risks in stone setting that particular stone, or style of setting.
Basically, I covered my ass, so that, in the rare event that I chip a $20,000 stone, I didn't have to eat it, for a job I was only making $500 on.
In your case...what if your card fails and you have zero image to give him?
What if anything happens that prevents you from delivering images?
In my eyes, I don't care if you collect the money after, or before, you are entering a service agreement. Even if it is verbal.
This debate was argued here not too long ago. Nobody here is a lawyer, so I suggest you consult one. Consultations are typically free.
Or just wing it, because, you know, nothing bad ever happens to good people.