Guide
YouTube thumbnails: WebP vs JPG
YouTube encodes many stills twice. JPEG is the historic path. WebP is often smaller at a similar look. Neither is a higher resolution than the other for the same filename family.
Two directories, same ID
/vi/{id}/maxresdefault.jpg and /vi_webp/{id}/maxresdefault.webp are siblings, not a 4K upgrade. If maxres JPEG is missing, maxres WebP is usually missing too. The downloader lists both when they measure as real images.
When to keep WebP
Web pages, newsletters, and app assets that already speak WebP. Smaller bytes, same 1280×720. The converter can also emit WebP from a JPEG original in the browser if YouTube never published a WebP twin.
When to keep JPEG
Print pipelines, old CMS, email clients that still choke on WebP, and any workflow that will edit in a tool that hates WebP. Re-saving WebP → JPEG is a generation of loss; prefer the JPEG original from /vi/.
Copy-paste both families
https://i.ytimg.com/vi/VIDEO_ID/maxresdefault.jpg
https://i.ytimg.com/vi_webp/VIDEO_ID/maxresdefault.webp
https://i.ytimg.com/vi/VIDEO_ID/hqdefault.jpg
https://i.ytimg.com/vi_webp/VIDEO_ID/hqdefault.webpHTML with a picture element if you host both (after you have rights to the still):
<picture>
<source type="image/webp" srcset="thumb.webp">
<img src="thumb.jpg" width="1280" height="720" alt="Video title">
</picture>Convert in the browser when you need PNG or a WebP YouTube never published: thumbnail converter. Path map: thumbnail URL format.