While there are so many photo editing app in the AppStore, the default iOS Gallery can do the basic editing like cropping. To crop a photo on iPhone 5s and of course other iOS devices, do as follows:
1. Open the Photos application on your iPhone
2. Find the photo you want to crop and open it
3. Tap the Edit button on the top right side to enable the photo editing mode
4. Tap the Crop and Rotate button in the bottom bar
5. Now just drag or resize the cropping frame and hit the Done button when you are finished
6. Your cropped photo is now saved as a new file and you may use it for contact's photo or wallpaper.