Alright i've been asked this quite a lot, so hopefully i don't have to respond to that question anymore after posting this 

1. First of all you gotta go to the crew's or your own Snapmatic album
The crews can be found here: http://socialclub.rockstargames.com/crew/spitting_llamas/games/gtav/snapmatic
My own is here (replace 'nicoshaka' in the url with your own social club name): http://socialclub.rockstargames.com/member/nicoshaka/games/gtav/snapmatic

2. Now click on the picture you want, and right click on it after that to copy the URL like this:


3. To paste it on the forum you obviously first want to hit the 'reply' button. Now you want to hit this small image of Mona Lisa in the top left corner:


4. Finally you want to past the url you got from Social Club between the 'img' and '/img':


5. Profit