YouTube wants a 16:9 thumbnail at least 640 pixels wide and recommends 3840×2160. The file must stay under 2 MB when you upload from the mobile app (50 MB on desktop). A 1280×720 JPG meets every rule and keeps uploads fast, so it is the default preset.
Photos from a phone or a design tool rarely fit: they are 4000 pixels wide, square or vertical, or saved as a large PNG. “Crop to fill” scales the picture until the frame is covered and trims the edges — best when the subject is centred. “Fit with blurred background” keeps the whole image and fills the sides with a soft copy of itself, which suits vertical shots and screenshots; it never cuts anything off.
Other presets: Shorts cover 1080×1920 (9:16), channel banner 2560×1440 with the 1544×423 text-safe area marked (YouTube’s safe area at the 2048×1152 minimum is 1235×338), and a square 800×800 profile picture, which YouTube shows at 98×98.
The quality slider controls JPG compression; around 90 % looks identical to the original. If a detailed image would still exceed the limit, smallyt lowers quality step by step and tells you the final value. Everything runs in your browser’s canvas, so nothing is uploaded.
Frequently asked questions
Will resizing make my thumbnail blurry?
Shrinking a large image keeps it sharp. Enlarging a small one (under 1280 px wide) can look soft, so start from the largest version you have.
Why JPG and not PNG?
JPG keeps photos small. A 1280×720 PNG can easily exceed 2 MB, while a good JPG is usually 150–600 KB.
What size is a YouTube banner?
Upload at least 2048×1152 pixels (16:9); 2560×1440 is recommended. Keep text and logos inside the centre safe area — 1235×338 px at the minimum size — and the file at 6 MB or less.
Can I make a Shorts thumbnail?
Yes. The Shorts preset makes a 1080×1920 (9:16) image. YouTube currently lets you add custom Shorts thumbnails only in YouTube Studio on a computer.