Once I found one I like how do I chose it to be my thumbnail I only get three choices one I want ain't thereYou can either screenshot a still frame and you can usually find a really good one then just put that as your thumbnail or you can create one in photoshop for example. The size is 480x360 but has about a 20x20 boarder top and bottom
I think you have to be a youtube partnerYeah I dont think I have ever seen the Custom thumbnail button. Do you have to have a certain amount of subs before you can use it?