Image in Post Content - How to make responsive?

Hi All,

On the 'Events' page of my site in mobile, you will see the images (which are just images placed inside a normal post in the WP dashboard rather than in the upfront editor) are not responsive. Yes, I have given them 'custom size' dimensions but even choosing 'Large' or 'Medium' doesn't change things for me - how can I make it that these images adapt to fit inside a mobile screen rather than now where they run off the screen?

When I tried to change them to 'featured image' instead, the trouble then was that they don't take up 100% of the available space on desktop, but they do fit on mobile. Catch-22!

Help greatly appreciated