what are the rules regarding what you can create a billboard about? i've searched but can't find anything...

It must be PG-13 and follow general site rules, such as not advertising another petsite. =) It's pretty flexible.

Would it be alright if I linked it to this:

http://www.dearbornmuttstrut.com/dms2/participantpage.asp?fundid=191&uid=263&fkroledescid=5 Like, asking people to donate money to an organization? Sorry if I sound stupid, I've never made a billboard before :)