Blurry screenshots

I have just bought a new iPhone 6 and it works absolutely amazing. The only issue i face is that the screenshots look perfectly fine and crystal clear when i take them but when i put them up somewhere,say,on my whatsapp or instagram story they appear dark on the top and pixelated and grainy not only on the receivers end but on my phone too. It is not even a week old phone. Kindly help me.