A slow crossfade through images, by Nitzan Hermon — always in sync with the gallery.
Mac — install Plash (free), add this page’s URL as its website, and it becomes your desktop.
Windows — add the URL as a web wallpaper in Lively Wallpaper (free).
Anything with a browser — go fullscreen and leave it running.
Options: ?interval=30 seconds per image, ?fade=4000 crossfade ms, ?fit=contain to letterbox, ?order=newest instead of shuffle.