PHP may limit upload file size to 2MB by default and will not let you to
upload a background image via the admin.
You can upload the image by FTP.
Upload the file to the admin/temp directory
Then, rename the file to background1.png (The number 1 is to identify that the background
is to be used with the first grid. Rename to background2.png if you want to
identify with the 2nd grid and so on)
You will be able to see the background in the Admin after it is uploaded.