Some applications request an awful lot of permissions when you install them. If you read through all the fine print you will see the apps can use your camera, microphone, and read your call logs and text messages. I have found pictures in my phone that I didn't take as well, usually in a folder associated with the offending app. I have occasionally seen strange screens when closing unused applications in the app manager. I would suggest a good antivirus program for your phone, and careful consideration of what apps you have installed.