MyThumbTester guide
Why can a listed YouTube thumbnail size fail in the tester?
A thumbnail size listed by the downloader was available during one browser check; the tester loads it again in a separate browser request.
A listed size is one successful check, not a permanent guarantee
MyThumbTester lists a public thumbnail size only after the downloader's browser image check loads it with usable natural dimensions. When you later choose Open in thumbnail tester, the tester makes a separate fetch for that selected public image, verifies that it received an image response, then asks the browser to decode it. A later load can therefore fail even though the earlier size check succeeded. The tester shows a specific recovery message instead of silently replacing the image. This is a reproducible availability boundary, not a prediction of click-through rate, ranking, distribution, or views.
Treat the listed size as an observed public response at the time the downloader checked it. It is not a promise about future availability, a quality score, or a claim that every browser will decode the same image at every moment.
The tester validates the selected public image again
The handoff provides the parsed video ID and selected quality name. In the tester, those values form the selected public image URL again. The tester uses a browser fetch with omitted credentials, requires a successful response, requires an image content type, then decodes the returned blob into an image before it renders the six local layouts.
If any of those later steps fails, the defined instruction is to return to the downloader and choose another available size. This is safer than presenting a stale placeholder as if it were the selected source image.
Use a short recovery sequence
First, return to the downloader and submit the same public video once more. Choose one size that the browser lists at that time. Then open it in the tester again. If the selected image still cannot load, choose another listed size or use a different public video for the layout check.
You can also use Download JPG for a size that the browser can fetch, then load that local file directly into MyThumbTester. A local image load and a public-image handoff are separate browser paths, so do not describe one result as proof that the other will always succeed.
What this recovery check cannot tell you
A later image failure does not establish that the source was removed, private, licensed for reuse, or unusable in every context. Nor does a successful retry show how YouTube will crop it everywhere or which version will earn more clicks, rankings, distribution, or views. It only helps you distinguish the initial public-size check from the later tester image load.