My “Finder” on my MacBookPro looks like this:

Note how I have custom icons for my top-level folders rather than then default blue folder icon.

Here’s how you can create something similar.

• Have the images you want to use as the folder icon “substitute” ready to go. I create mine in Canva. 
• Right click on the folder whose icon you want to update. Select “Get Info.”
• A window will pop-up, with a blue folder icon in the upper left corner.
• There are two ways to update it with your new image:
  • Drag and drop the .jpg or .png onto the blue folder on the pop-up window
  • Copy the image you want to use; click the blue folder on the popup window, and then paste

If I’ve lost you, Apple has a step-by-step tutorial on their Support website with screenshots.

Extra Tip:

To delete/remove the custom icon, simply bring up that same “Get Info” icon, click the custom icon, and hit delete on your keyboard. This took me a long time to figure out. 🙄