how do you post thumbnails for youtube vids, such as http://www.youtube.com/watch?v=Y69kGmPeHw0 ? thanks in advance.

Do you mean embedding them?

If so, on the Youtube page, click Share, then Embed, and copy the embed code and paste to the forum.

Like so...
<iframe width="560" height="315" src="http://www.youtube.com/embed/Y69kGmPeHw0" frameborder="0" allowfullscreen></iframe>

we also have native youtube tags like this:
if you stick the part from your original link after the = sign between them (leave out the extra spaces I put in):
[ y t ]Y69kGmPeHw0 [ / y t]

you get the same thing:

but apparently in a slightly different format than the embedded way.