could be both, but my browser console spits out warnings like
so there are some css-issues (pretty complex setup, really, with lots of external css-files and also quite a bit inlined css).Unexpected end of file while searching for ';' or '}' to end declaration. Unexpected end of file while searching for closing } of declaration block.
but specifically for the background image (which is in inlined css); try to add "custom-background" to the css optimization exclusion list and see if that solves your problem.
hope this helps,
frank