What are the precise best pixel dimensions for the header image in a manual?

What are the precise best pixel dimensions for the header image in a manual?

Manuals are a favorite feature of ours. We use them publicly and internally. We constantly struggle with the header image. We would be very grateful to know the exact pixel proportions that will look great on any screen size. 

Related problem: The thumbnail image for manuals does not populate in the Android app which makes the user's experience feel under-developed.

Please advise asap!

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