This question has already been asked in a different situation.
I am trying to achieve a seamless photo grid with jQuery Masonry.
Check out my fiddle:
http://jsfiddle.net/drw158/sAkjv/
There is a vertical gap. Why?
Display your images as block-level elements:
.ct-coll-item img {
display: block;
}
See if this works: http://jsfiddle.net/drw158/sAkjv/1/
As for the why, I have no idea. My guess would be because the images are inline elements, but that isn't a direct explanation.