The XML tag is reloadableImage, and the type is used as the filename. Do you want to use the same image for all games in a collection or a different one for each game?
As an example, the default Aeon Nox theme uses fanart as a type, and thus as a file name as well.
As an example: for Arcade in the Main menu, it will look for the following files:
- collections/Main/medium_artwork/fanart/Arcade.png
- collections/Arcade/system_artwork/fanart.png
- collections/Main/medium_artwork/fanart/default.png
For 1942 in the Arcade menu, it will simply look for:
- collection/Arcade/medium_artwork/fanart/1942.png
- collection/Arcade/medium_artwork/fanart/default.png