youtube-download-your-thumbnails.user.js

// ==UserScript==
// @name         YouTube - Download Your Thumbnails
// @namespace    https://github.com/LenAnderson/
// @version      1.0
// @author       LenAnderson
// @match        https://studio.youtube.com/channel/*/videos/upload?*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=youtube.com
// @grant        GM_download
// @connect      img.youtube.com
// @run-at       context-menu
// ==/UserScript==

(function() {
	'use strict';

	const $ = (root,query)=>(query?root:document).querySelector(query?query:root);
	const $$ = (root,query)=>Array.from((query?root:document).querySelectorAll(query?query:root));

	const download = async(details)=>{
		details.name = details.name.replace(/\?.*$/, '');
		return new Promise((resolve,reject)=>{
			details.onload = resolve;
			details.onerror = reject;
			GM_download(details);
		});
	};

	const run = async()=>{
		await Promise.all($$('ytcp-video-section-content ytcp-video-row a#video-title').map(it=>{
			const id = it.href.replace(/^.+\/video\/([^\/]+)\/edit.*$/, '$1');
			const title = it.textContent.trim().replace(/[^a-z0-9 \-\(\)]+/ig, '_');
			return download({
				url:`https://img.youtube.com/vi/${id}/maxresdefault.jpg`,
				name:`YouTube Thumbnails/${title} [${id}].jpg`
			});
		}));
		alert('all thumbnails downloaded');
	};
	run();
})();