SetFolderImages.swift

import Cocoa
import Foundation

let pathToImage = "/some/path/to/an/image.png"
let pathToFolders = URL(fileURLWithPath: "/Some/Folder/Path/")

guard let image = NSImage(contentsOfFile: pathToImage) else {
	fatalError("Unable to load image.")
}

do {
	let fileManager = FileManager.default
	
	let directoryContents = try fileManager.contentsOfDirectory(at: pathToFolders, includingPropertiesForKeys: [.isDirectoryKey], options: FileManager.DirectoryEnumerationOptions(rawValue: 0))
	for subPathURL in directoryContents {
	    if (try subPathURL.resourceValues(forKeys: [.isDirectoryKey])).isDirectory ?? false {
			Cocoa.NSWorkspace.shared.setIcon(image, forFile: subPathURL.path, options: NSWorkspace.IconCreationOptions(rawValue: 0))
		}
	}
} catch {
	print("Something went wrong...")
}