I never work on a project for a client without an agreement. The agreement includes an attachment called Scope of Services for each project. This spells out specifically what I will do and what it will cost. Anything beyond that is outside of the scope of the project and the cost is in addition to the estimate defined in the Scope of Services.
Scope creep is a common problem with projects and there are clients who will intentionally try to get much more from you than what you agreed to give them. I always try to give a client more than they expected, but the agreement helps to draw a line when you need it due to an abusive client.
David, if you already delivered the first PSD conversion, or the first two, then stop working on this and tell the client that you have already delivered what you agreed to deliver. If you provided a quote for 5 hours for one PSD conversion, it is reasonable to assume that 4 PSD conversions will take 20 hours. If they get stinky, then give them the $700 worth of work that they paid for and walk away.
"It's inexcusable for scientists to torture animals; let them make their experiments on journalists and politicians." -Henrik Ibsen