A friend of mine was having a problem getting rid of the photos on her iPhone 4s the other day. She had already backed them up to the cloud and on her computer but just could not seem to get rid of them on her phone. Here is the solution to clearing out the old and making room for the new.
Go to the folder where you have all of your photos stored
Create a new folder inside that folder and leave it empty
Open up iTunes
Plug your iPhone into your computer through USB
Go to the photos tab on your iPhone
Browse for the folder you just created
Your photos are now removed from your iPhone.