Image sizes in Posts

I would really like my images to be displayed in the same width/sizes as the width of the whole post, ot the same width as the Feature Image. Currently even though the uploaded images are large, they are displayed as small photos in the actual post. Can someone please help with a solution. Many thanks!
Eg. of a post on site: http://dreamtimetraveler.com/dreamtime-living/journey-to-the-living-descendants-of-the-incas-part-2/