ATTENTION for WP Version 5+: This plugin does not work with Gutenberg (no problems with Classic Editor plugin though)! Gutenberg brings its own HTML structure for oEmbed elements, which is quite difficult to target. This plugin checks, if the first line of the content is a valid oEmbed URL. If that is the case, it replaces the thumbnail image with the oEmbed element (e. g. with a YouTube video). Now you don’t have to care too much about a thumbnail image, since instead you would see the video or any other oEmbed content. You can even e. g. watch videos on your blogs front page without opening the post. Works for all WordPress contents, which have a thumbnail set and are not password protected. The active WordPress Theme must use the_post_thumbnail() or get_the_post_thumbnail() functions for this to work!