iPhone 7 PLUS photos become blurry when uploaded on social networks, zoomed in, zoomed out, cropped or not cropped, still the same thing

I am a first time user of an iPhone which i bought at iStore at Menlyn on the 20th of October 2017, It is Iphone 7 PLUS. The phone is giving me problems when it comes to uploading pictures on social networks and putting a profile picture on WhatsApp. The pictures are clear on the phone but when i upload them on Facebook, Instagram and put the picture as a profile pic on Whatsapp, the pictures become blurry, even selfies that are not taken from a far distance, whether the pictures are zoomed in, zoomed out or zoomed in, cropped or not cropped they just become blurry once they go to social networks even selfies that are not taken from a far distance, they do the same thing.

I went to iStore at Menlyn and they checked the phone and they said they cant see anything wrong, they said they think that the issue is with the 3rd party which in this case is Instagram, Facebook and WhatsApp, and they (the technicians and the store manager by the name of Suhail Nabee) said there's nothing they can do except for giving me a Demo phone meaning the phone that has been used already or they can refund me my money with a 10% less of what i paid, like i don't understand because i know my friends and colleagues are using the same phone but the pictures come out the way they are on the phone when uploaded on the social networks.

i don't even have a month using this phone but they don't want to refund me my whole money that i paid, instead they want to cut 10% off or give me a Demo and they don't even wanna replace it with a new one. And another excuse that they have is that they have never came across with a situation like this and they don't know what to do because the fault is with the 3rd parties, now i don't understand because i am a client at iStore, am i the one who is suppose to contact these social networks or is it the iStore or Apple?? i really fail to understand that.

The treatment that i got from the Suhail Nabee him self as a manager was worst treatment ever, i mean how do you tell a client that you don't know what needs to be done when you know very well that the fault is between the phone and Apps not the client and apps or the store. And worse part even them they agree with me that they can see that the pictures change on the social networks but they still can't replace the phone instead they want to take 10% of my money for something that i didn't even use for a month.

Menlyn iStore is just ****