How do i change the thumbnail of my youtube video?

ive heard something like they give you three thumbnails to choose from but i cant figure out how to get to the thumbnails? could someone help me out please? thanks a bunch!

From your YouTube home page click the 'Account' link at the top of the page.
Select 'Videos, Favorites, & Playlists' under 'Manage My Videos'.
Click 'Edit Video Info' by the uploaded video of your choice and click any one of the three thumbnails displayed at the top of the editable information.
Click the 'Update Video Info' button and you're done.

Sometimes the change doesn't take effect immediately and you may have to wait minutes or hours for it show up.

You can only do this with your own original videos. It doesn't work with other people's videos that you've added to a playlist or favourites list.

you have to upload a video, when you do it shows a box with 3 "scenes" from the video, you pick the one that is gonna be a thumbnail for the video. to get a thumbnail for our channel after you upload a video its gonna ask you "do you wish to save this as your thumbnail" or something like that :)
Good luck