Most thumbnails are seen at the size of a thumbnail, next to twenty others. This listing composes for that size first: one subject you can recognise at a glance, contrast strong enough to survive a small phone screen, and words big enough to read without zooming.
Text is optional. If you put a short headline in quotes, it is drawn in large, heavy letters exactly as you wrote it. Keep it to five words or fewer; the model spells short phrases reliably and long ones less so. Without a headline, the image has no text, and you can add your own in any editor.
The output is 16:9, the shape video platforms show thumbnails in. Check any text before you upload, and do not use it to imitate another creator's channel or a real person.