If you are referring to photos taken by the iPhone in the Camera Roll album and not synched folders from the computer in iTunes, then there is an easier way to delete multiple photos than by deleting each one individually.
There is no Delete All, but you can go into Photos, select the Camera Roll album, touch the arrow on the lower left corner, then select multiple photos by touching (and checkmarking) each photo to be deleted and then selecting Delete and then confirm Delete Selected Photos.
Steve