Desktop browsers make it easy to save the selected image, but a little organization prevents confusion when several videos have similar artwork.
Select before downloading
Paste the direct video URL, request its thumbnail, and choose the required available resolution. The download should correspond to the current selection. Check the preview again if you changed the resolution after your first inspection.
Use a project folder
Save the image alongside a short source note containing the video URL and download date. A descriptive filename helps, but avoid relying only on a title that may change later. Keep the video identifier when possible.
Check the actual file
Open it in an image viewer and inspect dimensions at normal scale. If the save action was blocked, use the direct image link and your browser's Save image command. Keep the downloaded original separate from cropped or annotated copies. That distinction lets you return to the source without trying to undo edits in a flattened export.