what is s.ytimg.com?

There is a great explanation on CNET of the s.ytimg.com. It states it is a site where Google stores cookes for your Flash activity. These cookies can only be deleted by going to their site, given in the CNET article.

Just go to CNET and do a search on the site for s.ytimg.com

It is YouTube asking to store some Flash Player information on your system. It's normal, if you consider anything G***gle does "normal" (or Flash Player, for that matter).

You can block the cookie or simply don't use the evil that is Flash Player.