It sounds like you need to deactivate "Daydream Mode".
Depending on your device's settings, the "Daydream Mode" may be activated when the phone is the under lock screen, when it is charging or both. "Daydream Mode" allows your device to display current events, time and photos when it is under any of the above conditions.
To turn off "Daydream Mode" please doe the following.
1.) Go into "Settings".
2.) Locate and tap "Display".
3.) Now tap "Daydream Mode".
4.) Use the slider to turn it off. Or, should you not wish to turn it off, you can choose an image (or images) to have it display, or change from images to just colors.
Side Note: Having it on and allowing it to run images or colors will drain the battery faster.