owner of pictures

Sorry for the newbie question...

I try to write a small module working with the "cart". My objective is to provide a list of all pictures in the cart, grouped by owner (with their email address), to allow users to easily request the original picture to the authors.

Until now, my module register correctly with the cart module, adding the option the the dropdown menu, but I can't find in the API the way to retrieve owner informations of a picture...

Could someone give me a hint ?

Thank you

Robert