Hi, 

I have found that if I have the website disabled (ie offline) and I try to set a banner image or I have previously have a banner image set, and I try to save any edits to the bootstrap theme, the save fails - I get an error "image page not found" presumably it can't access the banner image file because the site is offline.

To put it another way, I can only save any edits to the bootstrap theme and there is a banner image set if the site is online.