Re-sizing images for a newspaper style theme

Hello. I’m currently setting up my blog using the WordPress theme “Rebel Magazine” (here is an online demo of the theme: http://ythv.info/rebel/). As you can see the first post has a larger width than the subsequent posts, which are ordered in a narrower two column layout.

I am looking for a way to be able to post large images in my posts so that they look nice at the top of the page and then when the post gets pushed down to the smaller width area, the pictures will be re-sized to accommodate this. I tried doing some research and all I’ve been able to gather is that this is impossible using CSS and that I’d need to do this using a plug-in. However, I have not been able to find a plug-in that can do this for me. If anyone can point me to a solution it would be much appreciated. Thank you.