Pictures Not Displaying.

Hi all. I've been using my P9 for a little bit now and do like it, on the whole. I especially like the camera (was the main selling point for me) but lately it has developed a fault. Not all pictures or videos can be displayed. One minute it was working and then I get an error when loading images. I'm new to XDA but did a little looking around before posting but couldn't find anyone else who was having a similar issue. Apologies if I missed it. I'm just about to do a factory reset to see if that solves the issue but wondered if anyone would know of anything else that may cause it? Could it be a 3rd party app or launcher?

Also, I'm tempted to root it and flash a custom ROM (done it previously to an old Samsung) and wondered if there were any good suggestions?

Thanks in advance.

Matt