Featured Image Action Hook

If you wanna catch the event before saving the post, you won’t be able to do this with PHP – setting the featured image is done with JavaScript (there is a hidden field and it’s value is populated with JS when user picks a thumbnail from Media Uploader. Also the thumb preview is rendered with JavaScript.) So WordPress hooks won’t be useful here, you need to look for a JavaScript event.

Good news is that you can still work with JS an wp.media API like this:

// listen for featured image selection event
wp.media.featuredImage.frame().on( 'select', function(){

    // get the selected image ID
    var thumb_id = wp.media.featuredImage.get();

    // there you go, thumb_id is the picked image ID
    console.log( thumb_id );
});

This would get fired every time a user sets a thumbnail from the Uploader.