What I would like to do is, using the Image field in core, to restrict uploaded images not just on resolution, but also on aspect ratio.
In other words, the image must be 2:3 width to height, and at least 200 by 300 pixels.
I know that Image Styles can manipulate the way an image is viewed. I'm not interested in that. Rather, I want to restrict uploads the same way that you can currently set min/max settings for resolution, I want to add aspect ratio.
My guess is that I'll need to write my own module to add a new check to Image's checks, but perhaps you know of a simpler way.