how to add mainmenu backgrounds

that's not a bug but I need help, so basically, I was making photos on beamng, and I thought "maybe I can put them into the main menu", but I put them into the right file, with the normal version and the blurred ones, and I modified this :

by adding a 9th file (with the same name I named the photo) it should work because there's nowhere any limitation to the amount of pictures we can put in the game, but it don't work, so I hope someone knows why, or how to make it work.