How do you use any thumbnail for youtube vids?

In the fail/win vids and girls kissing vids I always see them use shocking/attention grabbing pics that usually have NOTHING to do with the vid and usually not in the vid contents at all. Ive even seen them use hardcore porn stills for the thumbnail- one i recall of a girl with a dick in her ass closeup not sure how long it was up for tho . They dont even show up anywhere in the vids so how do they do it?

sorry i can not really understand what you are trying to say?

what is so hard to understand? i want a custom thumbnail for my videos. i dont know how much clearer i could be?