Spell effect models?

I can't seem to figure out how to assign a model file to a spell effect.
There's no field for it in the object editor, does this mean that I can't make new effects and have to overwrite existing ones? I'm lost here. Haven't done any editing like this yet, and I assumed it would be the same as unit models, but it's not.

if there are no field you perhapsshould proceed with triggers by creating special effect on unit...