This little script will enable you to take a screen shot(like in SA) by pressing F3 button (niko will animate mobile like he is taking photo from there). Pictures are saved in "\My Documents\Rockstar Games\GTA IV\Photos\" folder in sequence. Picture resolution is same as your game's and format is jpeg.
This scripts works with
GTAIV .Net Script Hook (C) 2009 Hazard (firstname.lastname@example.org)
Download from here
After downloading and placing GTAIV .NET Hook in GTA 4 main directory, place this script in "Scripts" folder.
Feel free to feedback for any sug.