Is the featured image responsive?

I have been learning highend & am starting to get somewhere with it but am wondering if the featured image is responsive, ie is it resized to smaller devices as i have used a image 1200 wide for the site http://www.allanthemsdisco.co.uk/
Could anyone advise?
I am also wondering if the facebook button in the top right corner can be made larger as i cant find anything to do that.
On the left hand side of the site there is a empty space where the sidebar was, i have deselected it so why is there still a space there? again any advice appreciated, thanks!