After a lot of searching the Intertubes and cussing (as community troubleshooting and bug tracking for QT are abysmal) I took a closer look at what the designer fabricated there:
With some figuring the truth in the failure of the designer came to me: it forgot the quotes, making this an invalid css, for which it does not seem to care in the GUI preview, but breaks in the actual application (quietly I might add, not even an error message, the image just isn't there).
The correct CSS is:
This raises the question: does anyone test at least the basic functionality of these tools? Crap..