If we are only going to use a cropped part of an image, why not allow that on upload, to prevent useless files on the server?

Inspired by the cropping feature in {{ transform }}, I'd suggest something like:

images:
    type: file
    allowed: [png, gif, jpg, jpeg]
    transform:
      width: 600
      height: 600
      action: smart

So using the above code in the fieldset file, the image would be also cropped to 600x600, instead of only resizing (as we already have the resize option).

Another useful feature would be a preview of the image being uploaded and the area to be cropped, so the user can drag the cropping area to where he wants it.