Category site - reducing the size of the post image

On my category site here http://www.heikoraschke.de/category/kreativ-arbeiten/ the size of the post images is really big. I’d like to reduce them but can’t find a way to do so. Could you please help me?

Maybe it looks pretty good when the images float left to the text on the category site. Is there a way to do this in this theme too?

The easiest way would be if you would upload smaller images - original size is 1200xx675 and as this theme is responsive you can't set fixed size on the image - it will scale down with device width.