A standard YouTube watch link contains a video identifier. ToolNivo uses that identifier to look for available thumbnail images, then lets you choose among the versions it can load.
Paste the complete link
Open the thumbnail downloader and paste the video's URL into the input. A watch link needs its video parameter; a link containing only a sharing parameter does not identify a video. If you copied text from a message, remove any surrounding sentence or punctuation.
Choose an available image
Select Get thumbnail and wait for the resolution list. Compare the displayed dimensions rather than assuming every video has the same largest image. The preview changes when you choose another available version.
Save and verify
Use Download image, then open the saved file from your device's downloads area. If automatic saving fails, use Open image in a new tab and your browser's save option. Confirm that the saved image belongs to the intended video before adding it to a project.