i played with ubuntu tweak tool to change my login screen background but i could't use custom pictures.To use my pictures i changed permission to usr/share/backgrounds folder. I search wikipedia and used every chmod command.At last i was able to copy my pictures to backgrounds folder.But now my login screen wallpaper does not appear.I tried setting default wallpaper but it also didn't work.Is there any way to restore default settings and get the wallpaper back on login screen?
Just right click the picture and select set as wallpaper to set it. If your pictures format is not accepted just use gimp to convert it to different format (preferablly as .png file some jpg files once set as default wallpaper dose not appear in login screen)and set it. see this