Edit in JSFiddle

<div class="container">
  <img src="http://www.warrenphotographic.co.uk/photography/cats/30081.jpg" alt="">
  <img src="http://iwallhd.com/stock/yellow-labrador-retriever-puppy-white-background.jpg" alt="">
  <img src="http://i.imgur.com/nZlaeSH.jpg" alt="">
  <img src="http://www.cuter.cn/wp-content/uploads/2013/08/1375453731.jpg" alt="">
  <img src="https://puppydogweb.com/wp-content/uploads/2015/05/puppy-wallpaper-dancing-little-dogs-.jpg" alt="">
  <img src="http://www.petsworld.in/blog/wp-content/uploads/2015/03/How-To-Make-Your-Puppy-Gain-Weight.jpg" alt="">
  <img src="http://www.dogster.com/wp-content/uploads/2015/05/dalmatian-puppies-04.jpg" alt="">
  <img src="http://indiabright.com/wp-content/uploads/2015/10/Gallery-of-Dogs-Balancing-Cupcakes-539445.jpg" alt="">
  <img src="https://s-media-cache-ak0.pinimg.com/236x/63/60/39/6360396daf54afd024d181b6567a8c28.jpg" alt="">
  <img src="https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSr7jvGrFyqNOWgqfMD5t5zgOOf66pfGLVM8Jv0Uj16gfufLQTF" alt="">
  <img src="https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcR24wc_yoDK_i3nm6o6IV77ggd9KzBiBR9wwBlU3-bIgIEz9bMT" alt="">
  <img src="http://justcuteanimals.com/wp-content/uploads/2015/01/puppy-love-cute-yellow-lab-puppies-pups-dogs-animals-pictures-pics.jpg" alt="">
  <img src="http://omgcutethings.com/wp-content/uploads/2014/08/puppy-love-005-08092014.jpg" alt="">
  <img src="http://www.dallasvoice.com/wp-content/uploads/2015/03/puppy-2.jpg" alt="">
  <img src="http://www.perseyspetgrooming.com/images/puppy.png" alt="">
</div>

$(function(){
	mosaicGrid('.container', 'img');
});

function mosaicGrid(selector,target) {
  var cols = [0,0,0,0];
  var allTarget = $(selector).find(target);
  if (0 === allTarget.length) 
    return;
  allTarget.one('load', function(e){    
      var pos = minPos(cols);
      var x = pos * 100/cols.length;
      var y = cols[pos];
      $(this).css({left: x + "%", top: y + "px", width: Math.floor(100/cols.length)+"%"});      
      cols[pos] = cols[pos] + $(this).height();
      $(selector).height(Math.max.apply(null, cols) );      
      $(this).off(e);
  }).each(function(){
    if(this.complete)
      $(this).trigger('load');
  });
}
function minPos(arr) {
  var min = Math.min.apply(null, arr);
  for(var i = 0; i < arr.length; i++) {
    if (min === arr[i])
      return i;
  }
}

.container {
  position: relative;
  width: 100%;
  height: 50px;
}

img {
  position: absolute;
}