Responsive Theme: Resizing Featured Content Image

Hi,

I am using the Responsive Theme and I am trying to change the featured content image on the home page. The image is the one located on the right, next to the default “Hello, World!”.

I have managed to input my logo using the help from this post:
http://ww.wp.xz.cn/support/topic/theme-responsive-inserting-image-on-front-page

However I can’t seem to change the size. When I make the image larger it either ends up being a poor quality or not showing up at all.

How do I fix this?

Thanks,
Mac