In the same way the photos were transferred to your iPhone - via the iTunes sync process. If you want to remove all photos transferred from your computer, deselect Sync Photos under the Photos tab for your iPhone sync preferences with iTunes followed by selecting Apply and a sync.
You remove photos synced to your phone the same way they got there, through the itunes sync process. Connect your phone, itunes running and under the photos tab, remove the checks from the albums/folders you don't want on your phone & hit the apply/sync button. They will be removed from your phone.