Make sure to check Custom Fields section under Screen Options Delete the url of Video from Video URL field Scroll page down till Custom Fields are shown and delete iframe key U..
Just a quick reference 16×9 and 4×3 ratio images. The 16×9 Ratio: 100 x 56 – not ideal (too small, requires image to be stretched) 200 x 112 – not ideal (too small, requires image to be stretched) 300 x 168 – not ideal (small, requires image to be stretched) 400 x 225 – more ..
Background on Thumbnails Fundamentally, one needs to understand the concept of thumbnails before we get into how thumbnails work in WordPress. Essentially, a thumbnail is a smaller version of a larger, higher-quality image. Thumbnails are used in many WordPress themes to provide both informational and aesthetic benefit to site. Newspaper themes / Magazine themes use thumbnails ..