If you want to remove an inserted photo of a card back, you can perform the following steps:
Press this button on the app bar of the game view to show the list of secondary commands. On a PC, you can use the menu of the title bar alternatively. This menu is more detailed than that of the app bar, so you can reach almost all the panes directly.
On a PC, you may use the menu of the title bar alternatively. In contrast to the app bar menu, the title bar menu is more detailed and you can directly reach nearly all setting panes.