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We normally have this in the property name actually to make it very clear
Big Image (1024 x 768)
You could do a separate validator for it as well to warn editors by implementing IValidate
http://henrikm.com/custom-episerver-property-validation/
http://www.markeverard.com/2014/02/
This shows an example of restricting image dimensions too. Happened to stumble upon it :)
For image properties on blocks or pages we show the Description for more information about the best fit pixels.
It will be shown by editing the page/block, but not when you create a page/block and that page/block has a required image.
Is there a way to show this Description by the Create event?