Sorry, I should have been more specific about how to fix the problem. The builds with the problem scaled the background image incorrectly and created a background.jpg file with black bars. The new version doesn't do that any more. However, the old, bad background.jpg won't get replaced with a good one until you tell the program to replace it by dragging a fresh background image into the Render window. Otherwise, it keeps grabbing the existing, bad, background.jpg that was generated by the older build.
Edit: Found another reason for the black bars to occur upon startup. Fixed in b005