desktop picture

[mms]

In OS X, if you set a custom desktop picture, then where in the System Folder is the picture stored?

I'm asking because I have a desktop picture that I like on a computer, and would like to use it on another computer as well although I don't have the original nor can I find the site where I obtained it.

Edit: I assume that it probably won't be in the System Folder, but each user's Library, since it is not systemwide.

[mmmdreg]

It stays exactly where you chose it from.

[mms]

But if you set a custom background and delete the original picture, the background is displayed even after shutting the computer down and restarting. I assumed that meant that there was a copy kept somewhere in the Library, although I couldn't find it. Then can anybody explain to me how this is donw?