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
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?