Edit in JSFiddle

var omm = ProbabilityAPI.ObservableMarkovModel;

var probabilityTable = [
    [.2, .65, .15],
    [.1, .8, .1],
    [.7, .25, .05]
];

//fake an enum for readability
var states = {
    animatedGIF:0,
    photos:1,
    video:2
};

var result=0;
var probabilities = [];
probabilities[0] = omm.getProbability([states.photos, states.video, states.video], probabilityTable);

probabilities[1] =  omm.getProbability([states.photos, states.animatedGIF, states.video], probabilityTable);


probabilities[2] =  omm.getProbability([states.photos, states.photos, states.video], probabilityTable);

for(var i=0; i < probabilities.length; i++){
   result += probabilities[i];
}
document.getElementById('output').innerHTML = '<p>Probability: '+result+'</p>';

<div id="output"></div>

div{
    margin-top:40%;
}