YouTube thumbnails are an important part of your videos so you must get it right. The best YouTube thumbnail size is a resolution of 1280 x 720. Alongside the ideal YouTube thumbnail size, your custom thumbnail images should not be more than 2MB. You can make use of an app to compress your file. The accepted image formats include PNG, JPG, and GIF. An aspect ratio of 16:9 is recommended as it is the most commonly used in players and previews. A deviation from these recommendations can lead to the video not getting uploaded.