First of all you have to upload the necessary CSS pictures to the Wild Apricot server. The easiest way to do it is via WildApricot WebDAV folder feature.
Second, link to these these images in your CSS.
There are 2 ways to do it:
CSS Customization in Wild Apricot backend:
Login.
Go to Settings -> Visual look and feel -> CSS customization.
Specify necessary CSS rules for HTML elements where you want to apply a custom background image.
Click [Save & close].
Sample code above results in the 'body' CSS element using "red flower" image background.
Edit user.css directly via WebDav.
Open your WildApricot WebDAV folder.
Download "user.css" to your computer.
Edit user.css
Save and upload user.css back to WildApricot WebDAV folder.
Now the site background is filled with the specified image: