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Name the pic "Aqua Blue.jpg" and drop it in /Library/Desktop Pictures/

You might want to rename the original one first.
 
If you replace the "Aqua Blue.jpg" with any picture you want, but it must still be called "Aqua Blue.jpg".

It is located in /Library/Desktop Pictures

I havent actually confirmed this works but read about it a little while ago. I would check now but i am in the middle of converting a dvd and don't want to waste all the work done so far.
 
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