Hi there -
The short answer is "the highest resolution you can".
Here's a screenshot from a tutorial that's in the making about what happens to image resolution when it's used in video. Here I started with a photo from a Canon 5D (4368x2912). It was cropped to 16:9 (4368x2457) to use in a video.
The screenshot simply shows the comparison between the original (cropped) image resolution & final production or disc creation.
Cheers - Tony
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