MyThumbTester guide
What should you do if Open in thumbnail tester cannot transfer a thumbnail?
If the session-only handoff cannot be saved in this browser, download a listed JPG and load that local file into MyThumbTester instead.
Download the listed JPG, then load that local file into the tester
If Open in thumbnail tester cannot transfer a selected public thumbnail, use Download JPG for a size the downloader currently lists, then open MyThumbTester and choose that downloaded file from your device. The direct recovery path is built into the handoff error message because the downloader-to-tester convenience step uses browser session storage. A browser can reject or clear that temporary storage even though the selected public image was previously listed. This is a reproducible recovery path, not a prediction of click-through rate, ranking, distribution, or views.
The downloaded local file follows MyThumbTester’s regular browser-local image path: it is read with a FileReader, decoded in the browser, and drawn into the selected illustrative layout.
Why this is different from a later image-load failure
The transfer problem happens before the tester navigates with a usable temporary request: the downloader cannot save the parsed public video ID and selected quality in session storage. A later public-image failure is a separate case, where the tester received the handoff but cannot fetch or decode the selected public image again.
Downloading a listed JPG gives you a local-file route that does not depend on the session-storage handoff. It does not turn the image into a private copy on MyThumbTester’s servers; the file remains in your browser and device workflow.
Use the local-file recovery sequence
Return to the downloader, submit the same public video, and choose a size that is listed at that moment. Press Download JPG. Then open the tester, select the downloaded JPG, enter the title you want to inspect, and switch through the six layouts. If the download itself fails, choose another listed size or try the public link again instead of inventing a direct image URL.
This sequence works only with a public thumbnail image that the downloader can list and download. MyThumbTester does not access private videos or retrieve source-video metadata for the preview fields.
What this fallback cannot establish
Loading a downloaded public image locally does not prove ownership, reuse permission, future availability, or how YouTube will render the image live. It also does not test an audience or select a winning thumbnail. It simply preserves a workable browser-local route to inspect one downloaded image and creator-entered title.