Here's one method:
Make a free account at www.photobucket.com
Upload images into photobucket.
Copy the link that begins with IMG (should be fourth under your uploaded image)
Paste here in a reply window.
Blam! Photos on ZS in 5 minutes for free.
Or you can follow same method using www.imageshak.com or www.flikr.com or any other number of free picture hosting services.
We frown on hotlinking here (pasting a picture's address between two IMG tags without first hosting it yourself) so take a second or two and make a free account and you'll be set.
If you have any further questions about posting pictures on Zombie Squad, don't hesitate to send me a PM.