First make the billboard itself in either Gimp or Photoshop. Make sure it has transparency if needed. Save it as a .png. Then open the material editor. You can make the billboard from scratch, but I find it easier to just open another already made billboard and then find the "diffuse file" box and switch in your picture. Also if you want your billboard to be solid (meaning you can't see through it) change the Blend Mode to Alpha. Then click File>Save As (Don't click "Save" because it will overwrite the one your editing) and put it the new one in your custom story folder. Then in the editor just go find it and bada bing.
This always works for me but I am by no means an expert, so if you still have trouble, just wait a while and one of the Super Friends will be along eventually