I can't change icons on my mac!

Hi!

I´m not able to change any icon on my macbook pro 15´. It´s a mid 2015 model. I am the only user/owner and also the administrator.

When I try to change the icons of eg. safari, I get the "error" sound. The way I go about changing the icon is that I open the "get info" folder of Safari, and try to copy a new .png file there. It does not work. I have also tried to edit the "read only" options on everything, but I´m not able to. Whenever I try to change those I get the "can't finish the operation because....don´t have permission" error or something.

What do I do wrong and how can I change this??

Thanks in advance for any and all helpful answers!

P.S.

I´m running OS X El Capitan.