2014 Theme feature image display

Hi,

I’m looking for help sizing the featured images on posts in the Twenty Fourteen Theme.

I used to run a wordpress.com website with that theme and the featured images resized automatically. Now, even though I have set it to width: 100%,they get cut off.

Example the way it was / how I want it:
http://fernwehandwanderlust.wordpress.com/2014/07/01/camp-nano-for-this-blog/

Example of what it is like at the moment:
http://www.fernwehandwanderlust.org/camp-nano-for-this-blog/

I also like that the text box on a post slightly overlaps the bottom of a featured image. How do I set this up so I can widen and centre the post? Example is the wordpress.com link above…

I want the featured image to span the whole width of the middle column and the text abou 80%, leaving a bit of the overlap and padding either side.

I’m brand new to CSS so fool-proof explanations would be appreciated!

Thank you