Moto CMS 3 base background image.

I have a CMS3 template that on PC view would be best described as having a base image as the various containers float over it.  The background image actually switches half way thru as the container items float over it.   How do I change the base float over background images.   I been looking around on your help topics to see solution but have not found anything yet.  I also click the various containers etc hoping to get into a edit screen type thingy. 

Thanks