Step 1 – Backup your photos.The first step I took is to back up all my photos to an external hard drive, I manage all my images using Lightroom 4 and store them on a single external hard drive. The way I did this was to use a piece of software called IExplorer. It allows you to use your Iphone like a USB drive. You can navigate to the folder where your photos are store by following this file path. Root/private/var/mobile/Media/DCIM/100APPLE Just select all the photos in this folder and copy them to where you want to store your backup.
Step 2 – Delete all the photos on iphone camera roll.Once you have your backup, you can safely delete all the photos on your iphone. The problem is there is no way to do this from your phone. I discovered a quick way to do this using Finder after searching extensively on the net for a solution. Thanks to tandoantd for this solution
connect your iphone to your mac
select file > import from iphone
select command + A to select all photos
hit the delete symbol on the bottomThis will take a while to delete all the photos especially if you have a lot, so take a break and come back in 20 minutes. Once all the photos are deleted you are ready to select the best ones and put them back onto your phone.