This question applies to RAB as well. (I already asked this question there a few weeks ago and got no response, but now I’ve been asked the same question for DAB).
In the DAB (and RAB) manuals it mentions _"Keep the image size small enough so that they display well on a small screen and will not make the app size too large. DAB will allow you to resize the images after you have added them to the app project."
What is the best size to use for images within an App?
I don’t think there is a right answer. If you include images that are 640x480, they might look great in portrait mode on the average phone, but be pixellated in landscape mode. It would look poor on a tablet either way.
960x720 would be be better, but a nexus 5x phone is 1920x1080 (possibly average for today) and the trend keeps going upward. On that screen, a portrait image should be 1080 wide, and a landscape image 1920x1080.
Larger images are easily scaled do down, but scaling up is when the pixelation starts.