licensing photos advice

[seemoo]

Hi

Someone wants to license some of my pictures and since it's the first time for me, I wanted to ask you guys for some advice.

This is parts of what I received:
"I would like to license your photograps to be used in a computer game for a specific one time fee. You would still keep the full rights and still own them, I would only like to have your permission to use them and edit them (colors/contrast/etc.). You'll also be listed in the game's credits.

We would also be working with a legal contract so that it's all professional. For the final delivery, I would like to receive a higher resolution of the photographs, preferrably in a CD."

I know the name of the Game Studio and it's for one of their upcoming games.

My question is how much can I charge per picture for it?

Thank you already in advance for all your advice.

simon