Can't change app icons on MacBook Air

My friend showed me that you can change the images of the icons on your MacBook, she showed me how, but when I tried it, it won't work. I looked it up and it's the same thing I'm doing. I'm copying the entire transparent image, and when I paste it, it just makes the error sound. I can't find any solution to it and I know I'm not doing anything wrong and every article I look at says the same thing. It's extremely frustrating so I'd appreciate if someone could give any advice.