help

IPhone 6 Plus iOS 8.3 not jailbroken

somehow screenshots of my text/email are being taken from my phone and sent to another person anonymously. I noticed they appear to be able to do it only while I am on wifi at home, I have changed my home ssid and wpa2 password several times, and my router has MAC filtering. sometimes the screenshots are of sms sometimes imessage. but they were also some screenshots of my emails. the screenshots look like they are grainy and not an actual screenshot from my phone. can anyone help?I did hear sometime back that the screen changing stores a cached image of the last thing open in that app that was open and it is vulnerable. also this was happening on the other ios versions as well.