You can make any jpg your wallpaper quite easily within OS X Leopard providing you have root access.  I have seen some people doing this using Terminal but it can be achieved quite simply using Finder. 

1. On the Go menu, choose Go to Folder… and then enter /System/Library/CoreServices
   
2. Copy the DefaultDesktop.jpg image into another folder.  
3. Rename the image you want to set as your background to DefaultDesktop.jpg and then copy this into the previous folder.
4. Logout and see all your hard work put into fruition

Enjoy

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