// ImageController
public function postImage(Request $request)
{
  $this->validate($request, [
    'avatar' => 'dimensions:min_width=250,min_height=500'
  ]);

  // or... 

  $this->validate($request, [
    'avatar' => 'dimensions:min_width=500,max_width=1500'
  ]);

  // or...

  $this->validate($request, [
    'avatar' => 'dimensions:width=100,height=100'
  ]);

  // or...

  // Ensures that the width of the image is 1.5x the height
  $this->validate($request, [
    'avatar' => 'dimensions:ratio=3/2'
  ]);
}