Sign in to your YouTube account and click on the gear icon in the top right corner to access your account settings.
In the Overview tab, click on the Verify button.
Follow the prompts to verify your account using your phone or email.
Once your account is verified, you can follow these steps to upload a custom thumbnail to your videos:
Go to the Video Manager and find the video you want to add a thumbnail to.
Click on the Edit button.
In the Overview tab, click on the Custom thumbnail button.
Click on the Choose file button and select the image you want to use as your thumbnail.
Click on the Save changes button.
Remember that your thumbnail image should be 1280 x 720 pixels and in a .JPG, .GIF, .BMP, or .PNG format. It's also a good idea to use an image that is visually appealing and represents the content of your video.