YouTube offers the option to view content in high definition with 1080p resolution — when the source upload supports it.
Also see;
How do you Upload Video in HD to Youtube?
For the highest quality, YouTube now recommends the following setting for your video:
MPEG4 (DivX, Xvid, H.264) format
1280×720 resolution or 1080p
128k Mono or 320k Stereo MP3/AAC audio
24,25 or 30 frames per second
Up to 2GB file size and 15:59 min. duration
YouTube accepts a wide range of video file formats such as .WMV, .AVI and .MOV but you may get the best results from converting your file to MPEG4 video with MP3 audio.
The upload limit on YouTube videos has been increased from 10 minutes to 15 minutes. One of the reasons we can increase the video time limit is that we've made incremental improvements to our Content ID technology that helps content owners manage their videos on YouTube. We look forward to increasing the upload limit as Content ID continues to evolve and become even more sophisticated.
Regular user accounts are unable to upload custom thumbnails for their videos, however, Youtube partners are able to upload their own custom thumbnails.
However, doing a little searching apparently there's a grey-hat way to do this for non-partner, but there's a pretty big risk in using this method since Youtube could ban your account: Custom Thumbnail for Non-Partners
Best Answer
Just upload them as usual, as long as the source material is in HD this is how it will be added to YouTube;
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=132460
Also see;
http://www.reelseo.com/youtube-hd-update/
Hope this helps.