Just put another object of the Background Image extension and on the option "IMG URL" under 'Background Info", just type '' instead of a full URL like the default "'http://4.bp.blogspot.com/-mHaluG7qA28/UCrZcw03t2I/AAAAAAAAEI0/bhccobL7vmM/s640/LANDSCAPE-BEIRA-MAR-1.jpg'".
You will want to delete the extension. On Desktop this will be a case of deleting the appropriate folder under C:\Users\[userrname]\Documents\Qlik\Sense\Extensions. In Server it needs to be done in QMC under the Extensions section.
Once deleted it should cause you now more problems!
Seems strange that you can not get rid of it without deleting it though. You could try contacting the developer?