I have a project composed of still images that are set at 1920 x 1080 29.97 fps.
1. Is there a forumla for deciding the bitrate? What do you use to determine this?
2. Which option should I select for the aspect ratio?
3. There is an option for NTSC or PAL. I thought these two options only applied to SD video.
OK, for 1920X1080 at 29.97 you should be using an aspect ratio of 1.0 - meaning that the pixels are square. If you create a project using any of the HD presets of 1080p30 you should be in great shape. Not any of the 1440h 1080v (1.3333). Those will just cause you trouble unless you have the footage to match. You can use 29.97, or you can use 24 if you wish. It depends on the look you are going for. There are people who swear by 24 for film like results.
If you mean the bitrate for your export, the bitrate depends on what you are doing with the images. If they are sitting still and you are just cutting from one to another you can use a pretty low bitrate. If you are panning and zooming, then you need a higher bitrate.
The real question is, what are you exporting to? If you are going to put the video on YouTube or Vimeo, use the presets provided. If you are going to embed it on a web site, you need to check the results of different bitrates and decide what makes you happy.
You should not be concerned with NTSC or PAL unless you are exporting to a standard definition DVD.