I'm having trouble visualizing the problem, but that's OK. I haven't gotten into this DVD stuff yet. Would it work to increase the size of your image canvas - maybe put the images on a colored or black background?
You could play around with File>New, set it to a resolution to match that of your images, and then drag/drop your current image on top. You'd have to play with the numbers to get the right size canvas, but then you can place the image centered, have the top and bottom margins larger than the sides, or ???
I've obviously never tried this, but it might work. Or, the software you're using just might resize everything and sabotage your efforts! Good luck. If this idea is worthless, repost and somebody else will come up with a different idea.