I don´t have C2 around currently, but you could set the animation frame to "int(random(x))". That generates a random number between 0 and X.
"int(random(x))" will generate a random number between 0 and X, but 0 and X will have 50% less chances to be generated comparing to all the numbers in between.
It's better to use "floor(random(x+1))" if you need a range between 0 and X.
OR
"floor(random(b-a+1))+a" if you need a range between A and B.