I am developing a website for a company providing content writing services, what image would you recommend to use for my header design?

A pen with a notepad? Although it might be trivial.

a pen and a notepad is great idea you can also add it up with a globe if you the site is providing content writing globally

I'm assuming the websites ( http://onestopmarketing.com/osm/ ). I would say go with something that shows your global. Maybe have the O in STOP be a globe. Not just any globe make it look different but make sure visitors can still understand its a globe. Good luck!