iphone 4s: I erased photos by mistake...what can I do?

If you have a backup of your phone that included those pictures, then restore to that backup

http://support.apple.com/kb/HT1766

then sorry your pictures are gone.

For future reference you should import your pics to your computer as you would a digital camera

http://support.apple.com/kb/HT4083 that way if you delete or lose the pics you can always sync it back via itunes.