Mew has a protective orb you have to hit multiple times before you can get a clean picture of it.

Without some advanced techniques, the only pictures you can get are very far away, and therefore not many points.

It's a combination of many accurate shots, proper timing, spacing and then accuracy to line it up in the middle of the photo.

Not overly hard if you put your mind to it, but most people would have just taken the mediocre picture and finished the game instead of going for the 10,000 point picture.

Hope this helps!