I'm afraid, Photos on CD & DVD 6 doesn't yet feature a function to manipulate the background color behind slides that don't fill the whole monitor. This is possible in the current version Xtreme PhotoStory 7, though. There you can adjust the background color and also access other effects that will help you integrate portrait/upright photos into your project without having to just show a section of them or having the ugly black borders on both sides. You might want to look into this new version.