I took this photo in the NYC subway a few days ago. My apologies for the quality, but I thought it's a great juxtaposition:
In the top of the photo is an ad from the NYC Department of Health, advising you to choose food with less sodium. (Here's an AP story about the ads.) But to the bottom right is an ad for McDonald's dollar menu, and those are everywhere. While it doesn't mean we shouldn't run such ads, it's worth remembering that the sheer volume of food advertising will always dwarf opposing health messages.