How do I restrict uploaded images to a specific aspect ratio? 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.
My advice would be to implement a custom file validator.

To see how these are used when saving files, you can check out file_save_upload. For an example implementation that is very close to what you want to achieve, check out file_validate_image_resolution.

My super quick attempt at this

/**
 * Validates an image upload as having a particular aspect ratio
 *
 * @param $file
 *   - the uploaded file
 * @param $aspect_ratio
 *   - the apect ratio in the format [WIDTH]:[HEIGHT]. E.g. '3:2'
 */
function file_validate_image_aspect(stdClass $file, $aspect_ratio = 0) {
  $errors = array();

  // Check first that the file is an image.
  if ($info = image_get_info($file-&gt;uri)) {
    if ($aspect_ratio) {
      // Check that it is smaller than the given dimensions.
      list($width, $height) = explode(':', $aspect_ratio);
      if ($width * $info['height'] != $height * $info['width']) {
        $errors[] = t('The image is the wrong aspect ratio; the aspect ratio needed is %ratio.', array('%ratio' =&gt; $aspect_ratio));
      }
    }
  }

  return $errors;
}