Also keep in mind there are two seperate ways to put an image on a macro. The one you're all talking about is /micon or /macroicon - when you use this method, you will get a blank M UNLESS you are able to use the ability. So, for instance, if you /micon "Blizzard II" as an ACN, but don't have that equipped as an Additional Ability, it will be an M. If you add it as one of your additional abilities, it will show the proper icon.

The other way to put a specific image on a macro is to right-click the macro on the "User macros" page and choose to change the macro image. This will give you the full list of icon images, and whichever one you pick will always be active regardless of your class or whether you can use the ability.

Unlike /micon, however, selecting your own icon image means that you don't get any of the active animations for that icon. For example, if you choose the "Cure II" image, it will not get the flashing dashed border whenever Freecure is active. The "Mercy Stroke" image will not light up when an enemy target's HP is under 25%. For abilities on cooldown, you won't see the cooldown clock.

/micon is functional, but limited in when you can use it. Selecing an icon is unlimited, but always a static image.