Thread:Disturbedfan1100/@comment-24182086-20140828113735/@comment-6812733-20140828153241

You have to confirm your account before you can make custom thumbnails Multi. You have to go to the Creator Studio (click on your profile pic in the upper corner) and then go to "Channel" and that's where the confirmation button is, I think... Then once you are uploading a new video, you can just click on the button "Custom thumbnail" (Again, I think) and then upload a picture from your computer to use as a thumbnail.

I have a finnish UI in Youtube so I'm not 100% sure if my advices are correct.