AutoWallpaper allows you to create different categories of wallpaper images and the ability to specify different categories to different monitors (windows version supports up to 10 monitors, Mac version supports a single display).

AutoWallpaper can resize your images, add borders, filter the image with up to 14 different effects, and create subtle gradients behind the image that match the colours in the image. You can even set up a category for Picture-in-Picture. Automatically cycle your wallpapers on startup, on program exit, every few minutes (up to 4 hours).