What i do it put a picture i want at the background, with the sky erased, in .PNG format, in the "my_objects" folder. Add the "alpha_rejection" code... actually just find the backdrops material & copy that, but give yours your own material header names and change the image names to that of your pictures. Save it is a .material file in the material file folder.
Go in object maker, type "flatplane.mesh" i believe it is, then change the material field to whatever the name you made was and save it as "backdrops(yourmapnamehere)" in the object thing, i'm told it works for other people without having to give ppl the material file too, but i give it out just to over-ride the default mesh texture, and you can change/upgrade the textures later.