Outdoor advertising (billboards, etc.)

Does anyone know if individuals using their own funds can place billboard ads promoting Ron Paul without violating FEC laws? Is there a difference between "approved by the campaign" and "paid for by the campaign" when it comes to the rules? I think it is very important to put Ron Paul in front of as many people as possible, because after we did the Manchester walk, we ran into people who haven't even heard of Dr. Paul.
The following are links to a couple large outdoor advertisers in case anyone else has an interest in this.
http://www.cbsoutdoor.com...
http://www.lamar.com/main...
http://www.clearchannelou...