you should be able to find your background image under appearance customize
i take it you are looking for your background image for the whole site
siteorigin css is not perfect but it is good and can locate most things
the other way is to inspect the site in your browser by right clicking and you will be able to find what class it is under...
good luck
phil

Hi, you may want to view below tutorial that could be of help to you How to Add Background Image Using CSS