Actually, I believe the above statement is ebonically incorrect. I believe that the "Ima" is short for "I'm gonna", so the statement would be:
"If Zimmerman walks! Ima (I'm gonna) riot!"
I'm not fluent in ebonics, but I've been watching the way blacks folks use their language on their twitter posts threatening white people, as they are outbreeding everyone else, so someday we'll all be speaking it.