This explains how to sync and/or remove photos from your iPhone. These are the instructions that they supply for removing synced ("My Computer") photos from your iPhone: Sync photos to your iPhone, iPad, and iPod touch in iTunes - Apple Support
if you want to wipe the data off completely, not just the photos. You can do this from the iPhone:
Open Settings
Tap on General
Scroll to the bottom and tap on Reset
Tap on "Erase All Content and Settings"
Enter your iPhone's lock code and tap OK to erase everything
Once it's finished, usually after a few minutes, it will be just as it was from the factory.