My phone has a picture stuck on the background?!

this is odd but I fell asleep while waiting for my friend to reply, then when I woke up I completely forgot about it. And when I found my phone, it was still on and open on the chat. But as soon as I closed the app, the picture of the chat showed up infront of everything. For example: I would open my home screen and still see the chat and what we had typed and everything. I turned off my phone and kept it for a whole hour just incase, yet nothing happened. I am very upset actually because this is the first real damage that happened to my 1.5 year old iPhone 6. Please help and thank you