How do I prevent image scaling when burning an iPhoto Slideshow in iDVD?

In iPhoto '11, I have created a slideshow and sent it to iDVD 7.1.2 (Share > iDVD...). iDVD does fine with maintaining the timing, the music, the transitions, etc. However, even though I have the project set to 16:9 and under iDVD's options I have disabled "Always scale slides to TV Safe Area" the images of the slideshow are stretched vertically, compressed horizontally. No toggling of any of these options has had any effect. Burning to DVD, "burning" to image, and preview all show the same thing: stretched/compressed images. This is not a problem of my player's aspect ratio, or my TV's aspect ratio. How do I prevent image scaling when burning an iPhoto Slideshow in iDVD?