MyThumbTester guide
Which YouTube thumbnail size is selected first when several are available?
MyThumbTester keeps the downloader’s fixed probe order and initially selects the first public thumbnail image that the browser successfully lists.
The first listed successful size is selected initially; you can choose any other listed size
When several public thumbnail files are available, MyThumbTester initially selects the first successful result in its fixed probe order: maxresdefault, sddefault, hqdefault, mqdefault, then default. The downloader checks each of those standard public filenames and retains only image responses that load with usable natural dimensions. After the results appear, it selects the first listed option for the preview; selecting another visible option switches to that option’s separate public image file. This is a reproducible interface behavior. It is not a prediction of click-through rate, ranking, distribution, or views, and it does not claim that the first option is best.
The first selected option is therefore a deterministic starting point, not an automated quality recommendation. The browser only knows which public responses loaded during that check and their natural pixel dimensions.
Why the order stays predictable
The downloader starts its checks from a fixed five-item list and preserves that list’s order when the browser’s results are collected. Failed or unusable image responses are removed, leaving the successful options in the same relative order. The first remaining item becomes the selected preview image.
For example, if the browser cannot load maxresdefault but can load hqdefault and default, hqdefault is initially selected because it occurs earlier in the tool’s defined list. This does not infer anything about the original upload or a viewer preference.
Choose deliberately before downloading or opening the tester
Read the measured width and height shown on each listed option, then click the specific option you want before pressing Download JPG or Open in thumbnail tester. Both actions use the currently selected option’s direct public image URL. If you want to compare two listed files, inspect each one separately in the six local preview layouts or download both and use the local side-by-side comparison.
Available sizes vary by public video. A size not shown in the list was not a usable loaded image response in that browser check, so it cannot be selected through this tool.
What the initial selection cannot tell you
The starting selection does not prove that a file is higher quality, licensed for reuse, permanently available, or displayed by YouTube in a particular way. It also does not identify the thumbnail that will win an audience experiment. It is simply the first usable public image response in MyThumbTester’s stated probe order.