Every YouTube image size
A YouTube channel needs three images, and each is seen at a very different size.
Thumbnail
1280 × 720 px, 16:9 One per video. The same shape as the player.
Channel banner
2560 × 1440 px, 16:9 Keep text inside the center 1546 × 423 px safe area.
Profile picture
800 × 800 px, 1:1 Shown as a circle, often very small.
The YouTube banner and its safe area
The channel banner is the most confusing YouTube image. You upload 2560 × 1440 px, but the visible part depends on the screen: TVs show the most, desktops a wide strip, and phones a narrow band from the middle.
The part shown everywhere is the center 1546 × 423 px. Put your channel name, upload schedule or tagline there, and treat everything outside it as background that may or may not be seen.
The tool crops your image to the full 2560 × 1440 from the center, so a wide image with its subject in the middle comes out ready for that safe area.
Thumbnails and your profile picture
Thumbnails are seen small, in search results and suggestions. A close-up face, a few large words and strong contrast read at that size; small text does not. Make the thumbnail at 1280 × 720 and check it shrunk down before uploading.
Your profile picture appears next to every comment you make and under every video. A logo or a tight crop of your face at 800 × 800 stays recognizable when it is small.