The artwork selector can be used to select artist, albumartist, and album art. The first item in the selection grid will allow the removal of the current art, the second item will always be the currently selected art, and after that will be all the search results from the local storage and various online sources.
Toolbar Menu
Gallery
Opens the android gallery / image picker to select an image.
View Mode
Grid Size
The number of items per row in the grid.
fanart.tv
One of the artist artwork sources is fanart.tv. They request that users sign up for their own personal api key. Using your own api key will result in more up to date image results. Set your personal api key in GMMP with the 'Set Personal Key' menu option.