YouTube thumbnails are usually in a 16:9 aspect ratio and have a recommended resolution of 1280 x 720 pixels, but they can also be in other aspect ratios depending on the creator's preference. They often feature an eye-catching image or screenshot from the video, along with text overlays, graphics, and other design elements that help to communicate the video's topic and style.
Creators typically spend a lot of time and effort creating their YouTube thumbnails, as they are a critical component in attracting viewers to their videos. A well-designed thumbnail can make a significant difference in the video's performance and ultimately determine whether or not it gets clicked on and watched.