We can post Videothumbnails and profile thumbnails in to our youtube videos.
Method 1: Video Thumbnail
Login to your youtube account.Click on “My videos” in the upper right.
Locate the video which we need to change the thumbnail image.Click on “Edit Video Info”
Here we can see some video thumbnails.Click on the video image we would like to change and click on “Update video info”.
Method 2 : Profile thumbnails
Go to your account page and Click on your account > My account > Channel menu.
click on profile picture and click on “upload an image”. Click ” Browse” and locate a picture.
Locate the picture you want to upload.Highlight it and Click “Open” then click “Upload picture”