Browsers do not always handle cross-origin image downloads in the same way. ToolNivo offers a direct image link so you can still use the browser's own saving controls when automatic saving is unavailable.
Use the fallback deliberately
After selecting a resolution, choose Open image in a new tab. Check that the new page displays the image itself. On a desktop browser, use its image-saving command; on a phone, a long press may expose a save or download action. Labels differ by browser.
Verify the saved item
Open the downloads area and inspect the file. Confirm it is an image rather than a saved webpage or link shortcut. Check its dimensions if the size matters to your work.
Keep the original link
If your browser blocks the action, copy the video URL and try a normal browser tab outside an embedded app. You do not need to give a website account access or install an extension merely to test this fallback.