Is there a specific technique used for getting image previews from links?

When you post a link on reddit, an image from that link is displayed next to it as an icon (most of the time). Sometimes it's the websites favicon, sometimes it's an image preview from an imgur link, etc.

Do they use a specific, well known, standardized technique for retrieving preview data or do they just use a custom service to retrieve an image from the page and pick it based on some criteria?