Unlike the other API calls, this API sends back the thumbnail as a PNG file, not JSON. The HTTP response code indicates whether the screenshot was successful (200), or incomplete (404) or failed (404). If the screenshot failed or is not finished, a default image "Not found" is sent.
You can crop your screenshots. The crop is done first, then the thumbnail. You can take a 1024x768 screenshot, crop it to 768x768, and get it scaled down to 300x300.