You have a bunch of images that you would like to see as your desktop wallpapers. You are looking for an application that could rotate these images periodically.
Try Wally. Just specify the directory where the images are located, set the time interval, and go. Wally runs on Windows, Linux, and Mac OSX.
sudo apt-get install wally
Wally knows a lot more; for a more detailed description check out this post.
About a year ago, I used another application for this task called
wallpaper-tray. However, this package is gone from Synaptic, it seems it has been replaced by
Update (20110509) [Unity patch]
Wally exists in the system tray. However, Ubuntu 11.04 introduced Unity that comes with a new top panel and applications are not allowed to be there by default. You need to enable explications explicitly. If you want to continue using wally, here is what to do:
First, get your current settings for systray-whitelist:
gsettings get com.canonical.Unity.Panel systray-whitelist
Then, add your application to the above list with this command:
gsettings set com.canonical.Unity.Panel systray-whitelist "[ your_previous_list_here, 'wally']"
This tip is from Antonio, the author of wally.
If you want to download cool wallpapers, you may want to check out my Wallpaper Downloader and Rotator for Gnome. It can be used as a downloader and you can pass the rotating job to wally.