For Macs: read this!
First of all, save the .png file to your computer.
Then use this:click here!
website to convert the .png image into a .ico file. Windows icons use the .ico format, so this step is 100% necessary. When asked for the size check all the boxes.
Once you do that, go to the folder you want to customize and make sure you aren't in library view by right clicking and clicking the "open folder location" option.
If there is not an "open folder location" option, then you are not in library view and can just skip that part.
Right click the folder and go to "properties" and click the "customize" tab. Then click "change icon" and then "browse" and find the location that you saved your .ico file. Select the .ico and hit ok.
and remeber: feel free to use it