Thumbnail preview
Upload a thumbnail and type your title to see how it looks next to other videos, at the sizes people actually see it.
Thumbnails that work at a glance
Most viewers see your thumbnail on a phone while scrolling, often smaller than a business card. One clear subject, a few large words at most, and strong contrast between the subject and the background usually beat a busy collage.
Turn on the squint test. If you can still tell what the video is about when it's blurry, the thumbnail will hold up in a crowded feed. Grayscale shows whether your contrast comes from brightness or only from color.