Changing Mac Folder Icon
What you want to do is get the image that you are wanting to change the icon of your image. right click on the folder icon, and click "Get Info," it will load the information, and at the top, it will show the folder icon as it does here.



What you do is drag the new icon to where the folder icon next to School is. It will appear with a + icon next to your image, and let go. It will then change the icon of your folder.