Hi,
This is the way I use my own desktop pictures and wallpapers, without using iPhoto.
Drag and drop the pictures you want from your iPhoto folder to the Pictures Folder in your home user folder.
Go to System Preferences > Desktop & Screensaver > Desktop, and add ( + ) the Pictures Folder to the list of Mac Desktop Pictures folders.
Or you can create and name a new user folder in your Documents folder, just for the purpose of displaying or rotating pictures on your desktop, and add it to the list of folders in Desktop in System Preferences.
You can set your new folder to display a single desktop pic, set the size and view, or you can set the whole folder to rotate the pics in that folder every 5 seconds, or every minute, and so on.
I've found it easier to bypass iPhoto, to run pictures on the desktop, and I assume it will work in Snow Leopard.
I have not done the SL upgrade as yet, am having no major problems with the 10.5.8 at the moment.
Nan