Just tried to add background image to GB container but did not show up on page. I tested whether AMP plug-in was causing this and – if activated – it indeed does not work.
However, when I switch AMP plug-in off, then add background image (which did work), save it and reactivate AMP plugin, the background image does appear!
So I have found my own work-around but I would still like you to know or have a look at it.