The new contemporary theme is what's referred to as "responsive". This means that it automatically sizes and organizes content in a way that looks great no matter what kind of screen you're viewing it on whether a phone, tablet, or PC.
To make your images responsive:
- Insert an image into the page you are editing.
- Right-click the image and choose "Edit".
- Click the Advanced Settings tab.
- In the Advanced Image Settings section, look for the CSS Class field and enter a space then "img-responsive" (minus quotes).
Here is an example: