Either way should achieve what you want, but Deecee52's should be the easiest.
The following thread might help:
http://www.aximsite.com/boards/archi.../t-143411.html
It includes:
Other options are ThemeGenCE or MSCEInf ...
With ThemeGenCe, if you drag and drop the tsk in the program you will find the images in the application folder (tdywater.gif, stwater.gif...)
With MSCEinf which works like winzip you can drop files from the Files Tab the files you want to the explorer (like Winzip).
Both these programs are freeware.
The alternative is to create a nth theme for your phone from your PPC .tsk file.