To add a background image
Go to your backend and click "Extensions -> Templates -> Crafty " and click "General" tab and Turn "ON" Development mode.
Go to your site files and open "templates/crafty/less/style.less" and find the below code
And add background-image to the body tag for example
Finally, go to your backend and click "Extensions -> Templates -> Crafty" and Click "LESS to CSS" button.
After getting successfully compiled message, click "General" tab and Turn "OFF" the Development mode.
Let me know if any issues.