Background colour preference - changing default

I have a background image that I am presenting in a questionnaire but there is a gap around the outside of the picture. I wanted this 'gap' to be black (instead of the default purple colour), but now that I have changed it, it wont display the background image I have saved in the questionnaire. Can you help?

This sounds like MediaLab and not DirectRT so I've moved the thread.

So all you see now is black background and no image? If you change it back to the origional or some other colour does that have any effect?