from n900wallpapers.com i downloaded 4 wallpapers and the respective .desktop-file to use it as a panorama-wallpaper on all 4 desktops.

when i go to "change wallpaper" and click "add" to add a new 4series-wallpaper, i dont get to see the .desktop-file on the location i saved it at!
i created a folder named "wallpaper" where i saved all 4 single wallpapers plus its .dekstop-file, but the .desktop-file is not showing up.

how do i add all 4 wallpapers as a single panorama-wallpaper without having to assign them one by one to each desktop?