Once you have posed for 18 photos, the achievement will pop. While you are taking your 18 photos, make sure to share 10 of them which will unlock The Artist achievement. After receiving the achievement for using all six different world selections, you can now just select "Keep Playing" to re-take your photo. You do not need to save your photos and can keep taking a photo using the same world selection. This will still count towards taking 18 photos and will net you another easy achievement.
Note: A photo gets taken when a clapping noise is heard, not just the clapping motion.