For the longest time, my favorite wallpaper rotator was Wallpaper Master. It is literally one of my very favorite freeware, just because I'm not sure there is one thing I can think of I'd improve about it. It works smoothly, has different collections for images, and runs silently in the tray. It is an amazing freeware....for someone with one monitor. But it's not so great at handling two monitors (at least not the freeware version), so I tried to find another freeware to handle it. It was actually a difficult quest.
But eventually I came across John's Background Switcher. An interesting name, I'll admit, but it does what it says. It's an interesting alternative to Wallpaper Master; it's more internet-oriented, as it can download images from multiple services such as Flikr, Facebook, Google Images, or even an RSS Feed to use as wallpapers. It's fairly feature filled, just with features I don't really desire or use. What it is good at is multiple monitors. It can do the same wallpaper on all monitors, different on each, the same spread across, or even just one wallpaper on the main monitor. In addition, it also has a ton of configuration, arguably more than Wallpaper Master. It's got configurable keyboard shortcuts, things called "montages" which is a collage of pictures, how big the cache should be, and a ton of other stuff to make the program run however you want.
There are a few things I wish it did have. It does not have categories, so it's difficult to switch between groups that you might want. Also, you can't choose what specific wallpaper you want, which is doubly annoying for multiple screens. And lastly, it takes a good while to switch. Wallpaper Master was usually instant, whereas John's takes a few moments, which can be annoying at times.
Nonetheless, it's a great program, and I highly recommend it. But maybe you need a much simpler approach. Maybe you don't need a wallpaper rotator, you just want a tool to easily set different wallpapers for different monitors. I hear you. You're lucky I stumbled upon Dual Monitor Tools. One of the tools is Dual Wallpaper, which is just a tool to very, very simply set wallpapers for dual monitors. And the best part is, it supports both separate images and one shared image, so it is basically all you'll ever need when it comes to your wallpaper needs. Oh, and it's open source, which is awesome. Check out some of the other tools, they're very handy.
So that's about it. Three different programs that all rock in their respective jobs. If you only have one monitor and want to rotate wallpapers, try Wallpaper Master. It will blow your mind and you will love it. If you have two monitos and just want to set different wallpapers or one shared wallpaper, use Dual Wallpaper. It's simple and will do what you want easily. If you have multiple monitors and want to rotate images, use John's Background Switcher, or considering buying the shareware of Wallpaper Master. "What," you're saying, "Shareware? Have you gone mad?" No. I believe that software developers should be rewarded for the time and effor they spent creating a great software. Wallpaper Master is a great freeware, worthy of money. And the shareware has multi-monitor support, so it all works out.
I've considered making a Picasa album for all my wallpapers, that is, assuming I can get them all organized one day. Anyway, hope this helps your desktops look prettier.