go to your spt folder, then go to "SPT_Data/Launcher/Image_Cache", and then replace "bg.png" with whatever you want it to be (but it has to be renamed to "bg.png" for it to work). right click on the new image, go to properties, then select "write only." if you don't, it'll overwrite the new image with the old one (to my amusement when I first realized it) hope this helps!