UsingMac has a nifty tutorial on how to change the little “Flag” icon used in OSX’s Mail application. 
Using the built-in flag feature (Command + Shift + L) is a great way to bookmark/highlight an email, but for me the flag icon that ships with Mail doesn’t stand out enough. This simple tutorial not only shows you how to customize the icon, but makes the star icons you see in the image above available for download.
