So, for my Tellius & Archanea map I want to add images in the background, like Ted Riley's Spartan Battle Arena map.
(link: http://warlight.net/SinglePlayer?PreviewMap=18587)

However, when I upload the SVG, the images don't show. Does anyone know how to fix this?

That's because warlight can't handle images, and you have to convert them to vectors for warlight to pick it up. There's a tool for that in inkscape, and you could also find a few on the web that will do the trick.