How do I remove all the pictures off my Iphone 3Gs?

The Iphone has synched with my main computer and taken ALL of the pix off of it and there is no trashcan icon on the phone at all. AT and T said I could do it through Itunes, but all the Itunes says is that it can IMPORT pix from my computer. I want to keep the pix on my computer and take them all OFF the phone. Help!!!