I think this is a subject that has come up often over the last few years but not many people actually pay too much attention to. Unless of course you happen to be a gamer who complains as to why the fan created game project, based on his favourite hit title, is not available for download anymore or has been shut down.
This is something I have talked with my illustrator about recently, because she at the time did not have much experience with it as well, when we were deciding on how I could use the image. The reason I bring this up is because I think it fairer for both parties to understand why it is an advantage for both to make up a contract that clearly defines what rights you have to your work that you make for a customer, and which rights you as the customer have.
For example, in the case that the image becomes a smashing success, you don’t get a penny because you don’t have the rights as the artist. On the other hand, you as the customer don’t want the image, for which you just paid hundreds of Euros for, to suddenly be used to promote a different book or something else unrelated to what it was intended.
It’s not my intent to write too much about this subject, but I encourage you all to look up the topic and discuss these things, either if you are an artist making works for other people or you are paying to have something made.