to remove borders around post/page

If you go to http://songspursuit.com/eliminate-border-around-image/

I’ve inserted an image into the post itself. What I want to achieve is the image taking up full width and height of the entire post/page (yea, would like to do for both post and page actually).

Thanks in advanced!

p.s. I’m using Responsive theme (they’re having paid support for the moment, so yea- where unfortunately, I only have $5 to pay for support which is nowhere close to being enough) at the moment – but actually I’d like to do this with my other themes as well if it’s all the same editing-wise.