Click on the size of wallpaper you prefer. Then right-click on the image that appears in the new window and click on set as background.

1366 x 768	1366 x 768	1366 x 768	1366 x 768
1024 x 768	1024 x 768	1024 x 768	1024 x 768
1366 x 768	1366 x 768	1366 x 768	1366 x 768
1024 x 768	1024 x 768	1024 x 768	1024 x 768
1366 x 768	1366 x 768
1024 x 768	1024 x 768