How to change default desktop for all users

[Şerban S.]

[...] Can someone help me with this?

You can use any directory.
When you click on the "Folder" ComboBox, the last option is "Other...". Then you may choose a directory (folder) of your liking.
I always use a custom directory that resides on my data partition (different from /home!).
Still, if you use the /home (directory or partition) you can create a directory "Backgrounds" and place there your favorite images.
This approach prevents an overwriting in case of upgrade.

Below is a sample to help you get the idea:

Best regards, Șerban.