Just been trying it out I prefer it to Fox Tab, to add an image you edit the group > Additional Style Options. If you don't want the image to tile you'll have to make it large enough to fit your maximized browser window.
Now you are making me feel foolish, because all that I find when I use the Speel Dial context menu, is "Speed Dial Options", which opens a window with multiple tabs, but none in which that I can select an image to use as wallpaper. Apparently, you are referring to something else, but since I know almost nothing about FF, how about spelling it out so that even I can understand. EDIT: Nevermind, I found it.