Changing folder icons on Mac is super easy but not very obvious.
Get an image that you like. It's best that it's not smaller than 64x64px. Open it in preview.
Click 'edit' and then 'Copy' or simply press Cmnd+C.
Find the folder that you want to change the icon of.
Right click it and select 'Get info'
Click on the icon in the info panel
Click 'edit' and then Paste or simply press Cmnd+V.
And that's it :D You've successfully edited the icon of your folder.
If youre looking for icons you can use mine, which you can download here.