If your images are at least 630 in width, then they will automatically scale to fill the full width of the blog 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.
If you find that your images look blurry or pixilated, you will find that you might not have set your images to Original Size. This is especially important when using a post slider.
Note: Always select "Original size" when adding images to posts.
To set an image as "Original size", go to the post editing screen. Then left-click on any image, and select "Original size" 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.