Actually there is a way if you wanted to go through a bit of trouble but rather than rename them on your phone, If they are in your photostream, go to your photostream photo on your PC or MAC and rename them there and then resync them, to your phone and if you wanted to create a folder to put them in and name the folder. Go to the top where photos are in iTunes after clicking on your device and click that folder to add it to your photo app. I bit of a workaround.
I have each of my photos in seperate folders that i have selected and placed on my phone that way to keep organization. Good luck.