Looping images really randomlyI

I need a constantly changing background for an animation. I use different drawings of the same background and loop them randomly, to avoid this looping look. Unfortunalety when I use random there are always two frames who show the same image. I try to solve this with valueAtTime but I always get an error-message with this expression.

temp0 = random();

temp1 = temp0.valueAtTime(time  - .1);

if (temp0 == temp1)

{ temp2 = temp0 + .1; }

else

{temp2 = temp0}

[temp2]