Just ask him if you can get a copy of the picture he snapped the other day when he was driving by since you don't get a lot of pictures of you with the kids as it is usually you taking a picture of the kids. His expression should tell you whether he is total dumbfounded by your request or if he acts sheepish at having been caught or nervous or whatever.
Since we do live in a world we would rather we didn't the first thought that came to my mind was the fact this neighbour knows - and has access - to the child through his relationship with the parent. I would be thinking of some vary not acceptable reasons why he would want a picture of the child that he felt he needed to take and not get from the parents.