How to add custom thumbnails to YouTube videos
1. Start by going to YouTube Studio. Click your avatar in the upper right corner of the screen, then click YouTube Studio.
2. Click “videos” in the left hand column.
3. Click on the link for the video you’d like to change.
4. Click on the “custom thumbnail” button and choose your thumbnail from your hard drive. If you don’t see this button, verify your account (directions below.)
You must verify your account to be able to post custom thumbnails. Verification is easy. Just visit www.YouTube.com/verify and follow the prompts. YouTube will text you a code to verify that you are a human being (with a phone.) That’s it!