A playlist is a collection of videos. Its identifier does not by itself select the individual thumbnail you want, even if the playlist page displays several attractive covers.
Select the specific video
Open the intended item from the playlist and copy that video's direct link. Some watch URLs contain both a video identifier and a playlist parameter. In that case, the video identifier determines the image being requested.
Check for a collection-only address
If the link identifies only the playlist, return to an individual item. ToolNivo's downloader does not choose a random video from a collection or generate a combined playlist image.
Keep the context separately
If the playlist matters to your research, record its URL in your notes alongside the individual video URL. Name the downloaded file using the video identifier so later items do not overwrite it. This distinction keeps collection-level organization separate from the actual image source and prevents mismatched thumbnails in a research folder.