Selling image rights to a painter

[tmdphoto]

Anybody have any suggestion or recommendations about how best to protect yourself if you decide to give (sell) rights to one of your images to a painter? In this case, it is an oil painter who wants to use an image as a source document for a painting. He could sell the paintings, sell prints of the paintings, license other use, ie for advertising or marketing purposes.
Is there a good standard contract?
What are the important considerations?
What might be a reasonable selling price for the image?
Any reason not to limit rights?