We now have an easy way of generating credits files for your game project.
If you're using a piece of art in your project, create a collection (Collect -> New Collection on the main menu) and when you download a piece of art to use in your project, be sure to add it to that collection that you just created (you can do that from the left sidebar of any art page).
At the bottom of every collection there is a "Download Credits File" link, which you can click on to download a text file with the credit information for all of the art files in the collection.