How remove vidoes and photos from iphone 4s memory once it has been synced and all photos imported to iphoto and deleted from iphone camera roll

I have iphone 4s and imac. Have synced phone with itunes and imported all pictures and vidoes to iphoto which were then deleted from camera roll as prompted by iphoto. i have however no memeroy in iphone as 1000+ images and vidoes showing in the memory bar of iphone summary on itunes.

Must be an easy fix but I an't work out how to do it

Apple novice here so help appreciated

Stuart