Allow only bitmapped wallpaper
User Configuration\AdministrativeTemplates\Desktop\Active Desktop
Permits only bitmap images for wallpaper.
This policy limits the desktop background ("wallpaper") to bitmap (.bmp) files. If users select files with other image formats, such as JPEG, GIF, PNG, or HTML, the wallpaper does not load.
This policy is designed to avoid the Active Desktop prompt. When users select a wallpaper with an alternate image format, the system prompts them to enable Active Desktop. By limiting users to bitmapped files, the prompt is avoided.