Probably the right way to do this is to edit the login preference file...I don't know how to do that...
The easiest way is just to change the name of the file you want for your login background to "Aqua Blue"...make sure you spell it exactly like the aqua blue desktop picture that you normally see.
Others here, can probably tell you how to do it correctly, but I find the shortcut works well enough.
Best of Luck,
Simon