What are the recommended dimensions for a banner image on a page?
There is no cookie-cutter answer, a basic line is the original graphic's size should cover the horizontal length of the page without enlargement in HTML and its height not to exceed 1/8 of the page height with the vertical scroll bars intact. But this is not a fixed standard either.
If you are talking about banners which act as the same with logo or any introductory content:
It's necessary not to overflow the "above-the-fold" area of the screen. It's best that people see all the important contents of your page without scrolling down.
If you are talking about banners as images which are part of the content:
Due to the responsive nature of all websites nowadays (and I'm talking based on my experience and what I see on other networks), it's better to use a square-like dimension so that it will look full-screen both on mobile and desktop.
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