In an earlier post I showed how I set my screenshots to save in a custom folder rather than onto my desktop (I seem to take a lot of screenshots). I also shared a little camera icon that I made for it. One of the comments asked how I changed the folder icon, so I've made a 30 second screen recording to show how.
1. Go to web page that has the image you want
2. Right click image (or ctrl + left click)
3. choose Copy image
4. select folder you want to change the icon for (single left click the folder)
5. Press cmd + i keys together (opens the info pane)
6. left click folder icon shown in the top left of the info pane (it will get a blue highlight border)
7. Press cmd + v keys together (this is the shortcut for paste). You’ll see the image will have replaced the folder icon.