To make an image fill the full content width, follow the steps below:
Step 1. Access your Blogger dashboard and visit the Posts menu.
Step 2. Locate and click on the Post you want to edit.
Step 3. Click on the picture you want to edit, then click the Image icon as shown below:
Step 4. Select Original from the list.
Step 5. Click on the Update button located on the top right of your post editor.
The image should now fill the full width of your post container.