iDesk is great for that and very lightweight.
Unfortunately it seems its wiki is down (maybe forever?).
But a while ago I wrote some instructions to set it in antiX's faq in Brazilian Portuguese. Maybe you could translate it with google: http://sblinux.org/antix-port/#Como_...fundo_de_tela_
Below is a sample conf file for the rotating background form ~/Wallpaper every 1 minute:
To have a fixed image, leave 'Background.Source:' empty and put the image path in 'Background.File:'.
antiX, it seems, doesn't come anymore with idesk and its GUI config tool but I'll try and find out wich one it used.