I agree with FennelGiraffe that most people's image of a football field would include the end zones, but I also agree with Nicole, Nancy, and Ferret that when I hear football field, I think of the field and not the whole stadium.

In America, most high schools have a football field. It looks like your second picture. In fact, I had to look VERY closely to assure myself that that wasn't a picture of the field behind the school 2 blocks from my house. I think a lot of us are more familiar with these kinds of fields than stadiums.

The bigger point is, to a lot of people, 100 yards or 300 feet doesn't mean much without some sort of concrete example. If you told me to mark off 300 feet, I'd very likely be WAY off unless I realized, "Oh, that's about as long as a football field," and used that image as a guide. I still wouldn't be spot on, but as a former track runner (the track goes around the field, as in picture 2) I'd be pretty close.

A lot of people are like that. Concrete examples are good for giving a general idea of what's being discussed for those who may have a harder time conceptualizing things in the abstract or visualizing distances based on numerical values.