YouTube Thumbnail Sizes — Max HD, HD, Shorts and Live

Behind every “thumbnail downloader” is a naming convention: default, mqdefault, hqdefault, sddefault, maxresdefault, plus vertical frames for Shorts. Knowing the ladder tells you what is possible before you spend time on a request that cannot be satisfied — like asking for a 4K cover that YouTube does not publish. These pages cover each rung, the video types they apply to, and how to get the best real file when the size you wanted turns out not to exist.