What model of camcorder is your original footage coming from and how did you get this footage from this camcorder to your computer?
Are you saying that the video from the camcorder is a 720x480 MP4 -- or is it another video format?
If you look under the Edit menu and select Project Settings, what settings are listed on the General page?
This issue is most often the result of using video footage that is non-standard or uses non-standard specs like variable frame rates.
When you add the first clip to your timeline, is there a yellow orange "render" line along the top of the timeline, above the clip? This is an indication that the program is not able to match its project settings to your video's specs.
I neglected to say - quite important, actually - that the footage involved only consists of still photos (in full Ken Burns mode, here). I am having them move, change size and be cropped. It is the cropping that is the biggest culprit. I will, for example, crop both sides of a photo to just have the person in the middle show - and that will indeed be how it is rendered. But when exported as an mp4, the crop will instead produce a a rectangle that's to the left of the person.
Are your photos resized to no larger than 1000 pixels wide and 750 pixels tall (since you're working in standard definition)?
Have you turned off the Scale to Frame Size preference, either in the program's Preferences prior to importing your photos into your project or individually by right-clicking on each photo on your timeline and unchecking the option?