Ohhhhhhhh, OK!
To do this you will need to become really familiar with the drop command.
Definitely read Vektorboson's Drop tute over in the editor's depot if you haven't already.
The effect you see in the pic is most likely a "dot matrix" bunch of .2 diameter particles that have been painstakingly mapped out in x/y/z coordinates, over the 'head' of the units, to spell out "+15"
Or you could probably make and config a custom icon and have it called as a particle, but that would take more explaining than I could probably be capable of...
-Grendel