I have an idea to rick roll: Download the rick roll video, and open it with a video editor, replace 1:37 to 1:45 in the rick roll video to a spoofed picture, after you uploaded it to YouTube, it will become a rick roll video with a spoofed thumbnail!
The timeline of the new video will be like this:

0:00            1:00         1:37        1:45      2:00  ......
|       rick roll video       |  spoofed  |  rick roll video    |
|                             | thumbnail |                     |

Two problems:

1. There are already too many of these.[/*:m:2bbqm4wr]
2. According to an entry on the YouTube blog, the thumbnails aren't going to be chosen from the 25/50/75 points soon. Instead, an algorithm will decide on appropriate thumbnails.[/*:m:2bbqm4wr]

I still tried thumb spoofing, and it worked.

I'm just saying that it won't work for much longer.

It would be so much easier if YouTube just let you chose any frame in your video for your thumbnail. Then you could just put your random image at the end or something instead of just randomly in the middle.

YES.
But YT knows people will spoof if they do that.

I like how Nico Nico Douga does it. When you upload a new video, it chooses 3 random places in the video for a thumbnail. If you don't like any of them, you can click to get a new random trio. You can keep randomizing until you find one you like. After you choose it, however, it sticks permanently.