(setq SPIF_SENDWININICHANGE 2)
(setq SPIF_UPDATEINIFILE 1)
(setq SPI_SETDESKWALLPAPER 20)

(setq nconvert {C:\Program Files\XnView\nconvert.exe})
(setq params "-resize 1280 990 -canvas 1280 1024 top-center")
(setq wallpaper "default.bmp")

(setq jpgs (directory "." {\.jpg$} 1))
(setq n (% (/ (date-value) 86400) (length jpgs)))

(! (format {"%s" %s -out bmp -quiet -o %s %s}
           nconvert params wallpaper (jpgs n)))

(import "user32" "SystemParametersInfoA")

(SystemParametersInfoA
    SPI_SETDESKWALLPAPER 0 (real-path wallpaper)
    (| SPIF_UPDATEINIFILE SPIF_SENDWININICHANGE))

(exit)