You may want to make an image in a blog post the full-width of the container like this:

To make an image the full width of the post container, follow these steps:
- Visit the Posts menu
- Locate and click on the Post you want to edit
- Click on the image you want to edit
- Click on the pencil icon
- Change the alignment to center
- Change the size to Full Size
- Click on the Advanced Options
- In the Image CSS Class input, add "full-width"
- Click the Update button to save your changes
- Scroll up the page and click the Update button
The image will now display at 100% of the width of the post container.