Hey, i have extremely strange problem. When I change background image in lightdm's config file (to 1600x1200 png), i just get black background in lightdm. What's more, when I leave it at default lubuntu-default-wallpaper.png and swap that file, I get black background on lightdm too! It is just change-proof, it just doesn't want user to change that ugly 12.10 wallpaper to 12.04's one.... The only thing that works is when I change background to some color, like background =#f597ff etc. I tried every possible location of file, resolution, compression, extension and nothing. Sometimes instead of black background with some files i get random stripes from that image... Once one of the wallpapers that shipped with lubuntu worked, but when i changed to another one it became black background again.
Is it known bug? Could you see if you can change lightdm background image in lubuntu 12.10?