A YouTube Shorts link includes a video identifier that ToolNivo can inspect for available thumbnail files. The returned image may not match every cover treatment you see in the Shorts interface.
Use the direct Shorts link
Copy the URL for the individual Short, not a channel's Shorts tab. Paste it into the downloader and request the image. A channel page represents many videos and does not tell the tool which one you want.
Inspect the framing
Look at the returned preview before saving. A vertical video can have a landscape thumbnail file with borders or a different crop. The downloader saves the available image; it does not reconstruct the vertical video frame or remove embedded borders.
Choose the right source for editing
If you own the Short and need a different frame, use your original footage or editing project. Enlarging or cropping the downloaded file cannot recover content outside its boundaries. Keep the downloaded preview labelled separately from any new cover you create so the two sources remain distinguishable.