How to set your own thumbnails on Youtube
There is no way to upload your own thumbnail to your youtube video unless you are a partner. In order to be a partner you have to have a lot of videos, views, and subscribers. So for the average user you can't set your own thumbnail without tricking youtube by putting it into the video. It's actually pretty simple.
Youtube gives you three options when it comes to the thumbnail of a video. The three thumbs come from the first three quarters of the video. So if your video is 60 seconds long the first thumbnail is at 15, second at 30 seconds, and the third at 45 seconds. Or you can think of it as 25%, 50%, and 75% of the total video time, or so it seems at least. I have a couple video where this is not the case, but this seems to be somewhat true overall. I have read several comments that are saying that the thumbnails are now random but this method is worth a try. You can also place a 10 second or so banner in the video right in the middle so even if the thumbs are random you might catch one somewhere in the middle.
So if you want a particular thumbnail or poster frame, as they are sometimes called, to be displayed for your video you're best bet is to put it right in the middle of the video. Don't be one of those people who put images in just so people will click on it but something that has to do with your video. I can't tell you how annoying that is when you click a video and the thumbnail misrepresents the video. So annoying, and perhaps why YouTube only lets Partners upload a thumbnail for their videos.