Nah it's not against the rules. Most terrainers pride themselves in how good terrains they can make without image editing though... >.>
That effect does fit the terrain very well, but it is better to do it without
EDIT: One awesome effect for my favourite image editing program damn awesome, I tried it on a couple. Look what I made >_>
Void, I dare you to try and make this with glow doodads. DOOOO IIIIIIIITTTTTT