On the Daily Kos there was this lady talking about how she took a picture of her screen like you did. She did it because she wanted proof of how she voted in case the final screen showed McCain and she had to get the poll worker to change it. Anyway, she said that the guy in the next station heard her taking a picture and started yelling, accusing her, in front of everyone, of voter fraud. His allegation was that there is scheme where people are paid to vote a certain way, but they have to take a picture of their screen in order to get paid.
The poll worker told the man, like you already know, that there's nothing illegal about taking pictures at the voting station.