Yes Delphoma, you should use 468x60 px banners for your site, because its based on 960gs ( grid system ).
About the second question i cannot understand that much, maybe you can find some example or give us screenshot?
Kind Regards
Yes you can do that, but why to force the people to type useless description if they dont have.
Sometimes short is better, and some users to be attractive dont even post a description :) Its just user style
My suggestion: keep monitoring and keep your site safe.
Also maybe would be better if its added some kind of captcha on the voting page so it can be aviod cheating.
About the google dont worry, just reply and tell them whats going on - i had same problem with copyright stuff :)
Also make sure you are updating files from child, and if you are planning to make HTML E-mails make sure you code them correctly or your messages might be blocked from the spam filters
What kind of image ?
Maybe he uploaded his banner when he registered now you are hosting his banner on your site, and you could delete it.
Google is always serious, just read what clearly says into the first message you have recieved.