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Use the default theme for the ruleset whenever possible (exceptions are when the product itself contains a theme).
Have something interesting on each quarter of the image
Don't overload the screen and make it look too busy or cluttered
Take a screenshot bigger than 1920 x 1080 so you can clip the window frame off and be left with a final image that is 1920 x 1080.
Avoid stretching or shrinking the image along any axis.
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