HTML CSS - Basic Responsive Site

I am creating my first responsive website coding by HTML and CSS.

In a large viewport, the website is how I want it however on a smaller viewport, I am not able to accurately vertically align the 'portfolio' section of the homepage.

The Divs/Img's are a few pixels out!

I have uploaded my site to here and would be grateful if any of you guys can check the source code to see what's causing them not to align correctly when on a small viewport?

Help would be much appreciated!