If your images are at least 630 pixels wide, they will automatically scale to fill the full width of the blog post. However you may need to check the settings when uploading an image to a post.
Blogger automatically converts your images into various sizes. If you do not select "Original size" when adding an image to a post, Blogger will automatically select a small size. So even if you think your image is large enough, Blogger may have converted it without you knowing. To fix this, you should always select "Original size" when adding images to posts. This means that Blogger will use the largest possible image.
To set an image as "Original size", go to the post editing screen. Then left-click on any image, and select "Original" from the options shown below:
Even if this makes the images too big for the post editing screen, they will be scaled to fit the template perfectly. Blogger will automatically convert any very large images to 1600 in width, so you can upload images directly from your camera if you want to.