The ability to upload a custom thumbnail image for your videos is a feature limited only to YouTube Partners. However, this little YouTube hack will show you how you can get your own custom thumbnail for your video even if you are not a partner. Be warned that this video is for informational purposes only. We do not endorse nor recommend any practice that may violate YouTube's Terms of Service. If YouTube bans your account for grey-hat practices like this, it's not our fault.
Custom YouTube Thumbnail Images For Non-Partners
And here's our friend Ronnie's full video tutorial