This achievement is a bit glitchy but will unlock with persistence. You have to take pictures and then upload them to your Facebook account. You unlock the camera later on in the game and it shows up in one of the weapon slots. Simply zoom in and take a photo and then press and login to Facebook and then upload it. You may want to number each photo so you can track how many you have uploaded. Just keep doing this over and over it doesn't matter what rating the photo gets. After a while though it will say upload failed when it does this try dashboarding and retrying or leave it a couple of hours and retry.
It may take a lot more than twenty for this to unlock. I had to have 20 pictures uploaded on the same day for this to unlock but it may vary for others.