Many of BlackBird Images' retail clients use billboards to get their advertising message out. Photography intended for billboards is different from consumer photography; the goal is to create an eye-catching image that delivers a message... and that commuters won't get bored seeing day after day. (Effective advertising first has to capture attention, and then deliver the advertising message. And a billboard can't be cluttered - you only have a couple seconds to get your point across.) Here's a billboard we created late last year.
It works on a variety of levels, and most importantly enhances the customer's professional image. (Although it is a little disconcerting for models to see a 12 foot high image of themselves.) "Eye contact" is important - people naturally pay more attention when eye contact is made, and she definitely draws attention; this billboard was displayed in Virginia, but we've had people from other states mention seeing it.
This billboard uses a similar layout and has the same effect. (Consistency is important in advertising as you build a brand presence - customers should know it's your ad, even if the ad has changed.)
Next time you see a billboard, take a second and decide whether the photo and the design served the advertiser's purpose. Good advertising should deliver... not just look good.