For the purpose of this tutorial you should have "right click" enabled on your trackpad, or magic mouse. These settings can be adjusted in "System Preferences" then by selecting the trackpad or mouse icon. The option for "right click" is referred to as "secondary click" on a mac.
STEP 1
Locate the image file you want to use as your new folder.
STEP 2
Select that folder image and open it with the "Preview" app - this is an image app that comes preloaded with every iMac device. Click on the image while open in preview, Press "Command A" (this will select the whole image) - then press "Command C" to copy the image. Press the red X at the top left of the window to close the image preview.
STEP 3
Right click on a new or existing folder on your desktop, the one you want to change or customize - then select "Get Info"
STEP 4
At the top left of the "Info" window there should be a standard blue folder, select that folder with your mouse so that a light blue highlight appears around it, then press onto your keyboard "Command V" - this should paste the recently copied folder image into the Info window AND change the icon automatically on your desktop.
Before Paste
After Paste
STEP 5
Repeat the above process to change more folders on your desktop.
NOTE
To remove the custom folder from the updated one, repeat STEP 3 - then INSTEAD of pasting the image in STEP 4, simply select the folder at the top left and tap "Delete" on your keyboard. This will remove the customized folder and revert the folder back to its original state.