Identifying dragonflies from pictures can be a tricky task. Organizations like Odonata Central will not even consider photographs as a record of a sighting for many different species because they look so similar to a different species.
So if you want to take pictures of dragonflies the best tip that I can give you is to try and photograph them from as many different angles as possible. If you are photographing them to turn into a survey as a record the best thing to do is to net them and then photograph them in hand making sure to photograph distinguishing features.
Or if you happen to find a cooperative dragon, like this immature blue dasher, you can move around the subject taking pictures from different angles, in this case front view, abdomen view and profile.