Hi and welcome to the Forum!
Well you have the right idea here. When you place your finger on a photo, does it expand slightly? Wait for it to expand and without lifting your finger, move it.
If its not working the app may just be playing up, close it completely and then try again.
When an app behaves strangely it is best to close it completely and then relaunch it. From the home screen do the four finger swipe up or double press the home button to reveal your multi task bar. These are all the apps that you have opened at one time. Long press the app until it jiggles and a red minus sign will appear above it. Press the minus sign, this will close the app, not delete it, now press the home button to return to your screen.
