Where these thumbnails come from
YouTube automatically generates several thumbnail sizes for every uploaded video and serves them from a public, predictable URL pattern - this tool just extracts the video ID from whatever link you paste and builds those URLs directly, without needing the YouTube API or any login. "Max resolution" (1280x720) is only generated for videos uploaded in HD, so older or lower-resolution uploads may be missing that size while still having the smaller ones.
Why five different sizes are shown
Different uses call for different resolutions - a full-size thumbnail preview needs the largest version, while a small listing or a thumbnail grid works fine with one of the smaller sizes and loads faster. Showing all five side by side, from max resolution down to the tiny default size, means you can download whichever fits your specific use case without needing a separate tool for each.
Why a broken image for one size doesn't mean the tool failed
Each size is requested directly from YouTube's own thumbnail server - if a particular size genuinely wasn't generated for that video (most commonly "Max resolution" on an older, lower-resolution upload), that specific image simply fails to load while every other size still displays normally, since each is an independent request.
Accepting any common YouTube URL format
Standard watch URLs, Shorts URLs, embed URLs, youtu.be short links, and even a bare 11-character video ID are all recognized - so you can paste whatever link format you happen to have copied, rather than needing to convert it to one specific format first.