Embedly looks at the following attributes of the page and then ranks and scores the images.
- If the oEmbed type is photo the url of the oEmbed object
- The thumbnail_url of the oEmbed object if the oEmbed type is not photo
- The Open Graph og:image property
- The meta image_src tag.
- Any images found in the API response.
- Images ranked and pulled from the body of the page.
Read more about how Embedly chooses images: http://embed.ly/docs/response#images