Build a thumbnail from scratch, score one you already made, and learn the psychology behind why some thumbnails win the click and most don't.
Upload an image, drop in bold text, pick a proven color pairing. Export at 1280×720 when it's ready.
Drop in a thumbnail you already made. This checks it against the same rules the builder follows — file size, aspect ratio, contrast, and how busy it looks.
The psychology and mechanics behind the click, condensed.