The watermark is a small thumbnail image that appears at the bottom right corner of your videos when you play them from your computer. When you mouse over it, viewers can directly subscribe to your channel as you can see in the picture below :
What are the requirements for a good watermark?
- Square images, with a minimum size of 150 x 150 px and less than 1MB
- Transparent background rather than opaque, and a single color in the selected image
How to add a watermark
- Sign in to YouTube Studio beta.
- From the left menu, select Settings.
- Select Channel Branding
- Select Choose image and choose which image you’d like to use as your branding watermark.
- Select the display time for the branding watermark: 1. End of video: The branding watermark will appear for the last 15 seconds of the video; 2. Custom start time: The branding watermark will begin showing at a time you choose; 3. Entire video: The branding watermark will appear throughout the entire video.
- Save changes.
See also "Manage your Channel Branding" (Google Support)