I know a crude trick that works, even without AppleScript. I'm sure there is a better way to do this, but I'll report this method.
1. Create a new folder that will hold the current desktop picture, e.g., "Desktop Picture" in your home folder.
2. Copy a single image, e.g., ~/Pictures/sunset.jpg, to the folder from Step 1. Give it a generic name, e.g., picture.jpg.
3. In the "Desktop" preference pane, pick Collection: "Choose Folder..." and select the folder from Step 1. Turn "Change Picture" on and set a time, e.g., "every minute". I don't think it matters whether or not you pick "Random order".
4. Anytime you want to change desktop pictures, copy the new picture over the file from Step 2, e.g.,
cp "~/Pictures/mydog.jpg" "~/Desktop Picture/picture.jpg"
When the time you picked in Step 3 is next up, the desktop picture will change. You could even make a shell script to do the command from Step 4, e.g.,
setdtp mydog
