I was looking for brushes of barbed wire you can draw with, not just pat the photo of it on your picture. I couldn't find any (or am I blind?) so I made my own brushes
Slightly curved line looks fine, but be careful when drawing the wire heavily bent.
The brushes are free, so download them and use them and I will be happy!
They are made in Photoshop CS6 so I have no idea if they will work on older versions.