Randomly selecting stimlus depending on previous response

Hello,
I have faced numerous obstacles trying to build what seems like a simple experiment with MediaLab, and am turning to your expertise for help.
The study starts by asking participants to view a happy picture for ten seconds. After ten seconds have past, two buttons appear which are labeled as "same" or "different." The idea is that if the participant clicks on the "same," another happy picture appears as the second stimulus for another ten seconds. If the participant clicks on the "different," an unhappy picture appears as the second stimulus. This process needs to be repeated twenty times, so each participant will see twenty pictures during the whole experiment.
Right now, I am using the skip function to direct the next sequence, but as I have no control over what the participant will choose (the pattern may be same-same-different-different, different-same-different, and the list goes on and on) the experiment ends at a different place every time I test it.

Is there anyway to make MediaLab understand that each participant can finish the study only after they have viewed 20 pictures, which are guided by the participant's previous response? In addition, is there also a way to choose a random picture from a pool of "happy' pictures and a pool of "unhappy" pictures?

Thank you all so much for your help.