I'm trying to change the background to an image, but I can't find a way to do that. I've read that there's supposed to be a background image option somewhere in the Display tab of Game, but the only thing there is being able to change the colours of the background and the Margin.

There is a property called backgroundimage, but the control on the Display tab says "Margin image:".

If you mean the actual colour behind the text (which is likely to make the text hard to read if you put anything there), I don't think there's a standard way to do that. You would need to change it in your UI Initialisation script, using a line like:

JS.eval("$('#gameBorder').css(\"background-image: url('" + GetFileURL("imagename.png") + "')\");")

(that's off the top of my head, but I think I got it right)