go find/buy an smg or assault rifle with hella ammo, body armour and some sort of fire if you can get it, cruise to santos ammunation if you need to, then crouch strafe round the snitches house and shoot the CAR so it blows up, then the bodyguards/security guys will have noticed you so take them out (not hard) and enter the house, the snitch just stood there for me so the first time i shot him in the head which failed the mission because he couldn't be identified, so kill him with a few torso shots and take the picture.