Based on the wonderful addon:
KillTheHealer!
http://www.wowinterface.com/downloads/info19357-KillTheHealer.html#info
Description:
Players can select a personal assist (yourself or a party/raid member) and a skull will appear next to the assist target's nameplate.
Dungeons/Raids
This allows for coordinated focus firing for people who are too lazy to mark a skull in PvE. As a DPS in random dungeon queues, I can easily just target the tank and type /kill. Then I can quickly know what the tank is attacking just by looking at the nameplates. If there are a few or even tons of mobs with the same name, just mouseover all of them really quick and the add-on will eliminate the false skulls and reveal who the tank is really on.
Arenas/Battlegrounds
Even in battlegrounds or arenas, it can get hectic in vent calling out targets while a lot of other stuff is going on.
Say there's two holy paladins called "kljfkls" and "kslkdjfs". Obviously you don't want to attempt to pronounce either one of them and you can't say "switch to the holy paladin" because there are two.
Well, with this add-on, maybe you can say that and your group will know which holy paladin from the skull next to their nameplate. Sure there's the /assist macro but that takes up another binding the player has to come up with and get used to.
How to Use:
To assist a party/raid member, first target them then --> /kill
To clear assist --> /kill none
To "assist" yourself --> /kill player
For testing purposes (i.e. training dummies),
Use /kill focus
then set a focus target (/focus) and show nameplates to test functionality.
Note that unlike the other ones, this will put a skull over the focus rather than the focus's target.
Adjusting Size and Positioning
Open with Notepad:
World of Warcraft\Interface\AddOns\KillAssist\KillAssist.lua
You will notice you can change the icon's x and y positioning, as well as the size just above the image path. All is found on the first 20 lines of the file
The Future:
http://wow.curse.com/downloads/wow-addons/details/getthatdude.aspx
GetThatDude has the ability to mark raid icons on ENEMY PLAYERS and sends that data to other people who has the add-on as well. (Exactly like Blizzard's raid icon system for PvE, except it works for PvP)
This is different from this add-on because GetThatDude requires other people have the add-on as well. If you're looking to assist off somebody and get a visual representation of when they change target and where that target is WITHOUT ventrilo calls, you only need that person in your party/raid (and attention to the nameplate which conveniently shows their health bar as your melting their face).