Although this is a responsive theme you still need to follow the rules as outlined in CSS. Instead of assigning hard values to your images think in terms of percentages or fractions. An image assigned a hard value of 400px will break your theme when viewed at smaller screen sizes - than say a desktop or laptop computer; whereas the same image assigned a value of 100% won’t. This is because the image will resize to accommodate the smaller screen sizes.
This image is assigned a value of 40%.
When you add content to your site, there is an excellent editor to use. Just remember to think in terms of percentages and/or fractions. The editor will automatically read the size of your image and assign a hard value to it, but you will need to change that before you submit the article for posting.