A little CSS Guidance Needed

I am trying to simulate the Pinterest style with thumbnails, which float together to form a kind of mozaic. I have inserted a page link where you can see the way it is looking now. The css I have used so far is give below:

What css should be used to get thumbnails 4+5 to float upwards with equal spacing below thumbnails 1+2 ?

Thanks in advance for your help.

.crp_related .crp_title {
color: #000000!important;
position: inherit;
display: block;
bottom: 0;
padding: 3px;
font-size: 12px;
text-shadow: none;
background-color: white;
-webkit-border-radius: 7px;
-moz-border-radius: 7px;
border-radius: 7px;
}

.crp_related li:hover .crp_title {
background-color: white!important;
}

The page I need help with: [log in to see the link]