Your storefront will always have a Login Page and Checkout Pages. While you can't customize these as much as you can some other pages because of login and payment security, there are a few customizations you can do.
Navigate to Website and click Customize > Theme Customization.
Scroll down and you'll see the various sections on customizing your Account and Checkout pages.
For the Side Banner it's important to know just how it will be sized on different devices. We use this handy CSS code:
background-size: cover;
Cover makes it so a single image can work across multiple screen sizes whether it's mobile, tablets or huge monitor sizes. As long as you use the recommended resolution of 1130x1400px your image shouldn't be stretched but it will be cropped to ensure it works on smaller resolutions.
This means it's also super important to not put any instructional words in these banners. On smaller resolutions there's a high chance your customer will probably see the wording cutoff completely.