To download this image at full size, click on it.
It will open full size. Probably. Right click and save the image somewhere.
Then right click on your desktop. From the popup menu:
Properties > Desktop > Browse (to the image you just saved). Also set Position to Stretch.
Then 'OK'.
That's for a PC. I don't own a Mac, so you're on your own!
EDIT : 24th June. Updated with revised times/dates for Cardiff City and the two Swindon games.