Basically this guide will teach you how to set up Raspi2PNG, use a single command script
and set up a samba link to easily access those screen shots from a designated folder.

Step 1
First you need to get the program/dependency and compile the bin

sudo apt-get install libpng12-dev

git clone https://github.com/AndrewFromMelbourne/raspi2png
cd raspi2png
make

Step 2
Now you need to move raspi2png to your local bin and create a .sh script

sudo mv raspi2png /usr/local/bin

sudo nano /usr/local/bin/screenshot.sh

sudo chmod +x /usr/local/bin/screenshot.sh

copy and paste this

#!/bin/bash
raspi2png -p ~/RetroPie/screenshots/$(date +%m%d%Y_%H%M%S).png

save by "ctrl + x" then "y"

Step 3
now to create screenshots folder and add samba link

mkdir ~/RetroPie/screenshots

sudo nano /etc/samba/smb.conf

copy and paste this at the end of the file

[screenshots]
comment = screenshots
path = "/home/pi/RetroPie/screenshots"
writeable = yes
guest ok = yes
create mask = 0644
directory mask = 0755
force user = pi

Step 4
Restart Samba server and take your first screenshot

sudo service smbd restart

in terminal type

screenshot.sh

Step 5
Access your screenshots folder through samba

\\retropie

or mac os x
smb://retropie

Step 6
Clean up left over folder

sudo rm -r ~/raspi2png

[img]http://i.imgur.com/06griWS.png[/img]

Thanks
Herb (looking into adding it to retropie)
Rookervik (helping me debug the steps)