How to delete photos to free up space?
There is another trick how to free up some space:
1) make sure you have less than e.g. 3gb free space left
2) go to you iTunes Store and try to "rent" a movie which will require much more than 3gb. (most movies need 5gb or more).

once you try to rent, your "purchase" will fail but your device is cleaning up space. you will notice some Apps as "being cleaned" on your home screen.