Left click any of the below sizes, then right click on the window after it pops up and choose “set as background…”

1024×768 | 1280×1024 | 1680×1050

1024×768 | 1280×1024 | 1680×1050

1024×768 | 1280×1024 | 1680×1050

1024×768 | 1280×1024 | 1680×1050