MyThumbTester guide
Why can a listed YouTube thumbnail JPG fail to download?
A size shown by the downloader was found in one image check. Starting a JPG download makes a separate browser fetch for the selected public image.
The listed-size check and the JPG download are separate browser requests
A public thumbnail size appears in MyThumbTester only after the downloader's browser image probe loads it with usable natural dimensions. When you press Download JPG, the browser makes another request for the selected public image, converts that response into a local blob, and starts a browser download. That second request can fail even though the earlier probe succeeded. The tool then gives the defined recovery instruction to choose another listed size or try again. This is a reproducible public-image availability boundary, not a prediction of click-through rate, ranking, distribution, or views.
A listed option is therefore an observation from the original image check, not a permanent download promise or a statement about the source video's quality.
What the browser does after you press Download JPG
The downloader uses the selected option's direct public image URL, asks the browser to fetch it without credentials, and waits for a successful response before reading the image as a blob. Only then does it create a temporary browser-local download link named with the parsed video ID and selected quality key.
The public URL, video ID, and selected size are not sent as Analytics event parameters. The download is created in the visitor's browser; MyThumbTester does not host a copied image library or download the video itself.
Use a short recovery sequence
If the download fails, submit the same public video again and choose a size that the browser lists at that moment. Then try Download JPG once more. If it still fails, choose another listed size or a different public video instead of guessing a direct image URL.
You can also use Open in thumbnail tester for a listed size if your immediate goal is to inspect it in a local layout. That handoff has its own separate fetch and decode step, so a successful or failed JPG download does not guarantee the later tester result.
What a JPG download result cannot tell you
A successful or failed public JPG download cannot establish copyright permission, ownership, future availability, or how YouTube will crop the image. It also cannot reveal which thumbnail will gain more clicks, rankings, distribution, or views. It tells you only whether the selected public image could be fetched for a local download at that time.