It’s certainly not a requirement, but recommended. If you ever want to add functions or add CSS directly to the stylesheet this would be the best place to do it. If you just have a few minor CSS changes, these can be done under Appearance->Customizer.
For the featured images, I think you just want to change the gallery image size if you want the stacked image effect. Looks like yours is set to ‘thumbnail’ right now. The WordPress gallery insert should have a dropdown for image size.
I’m seeing your footer changes on my end so I think you may have found it, but in case you have not, you can change these under the “AE Templates” tab in your dashboard. This controls your footer across the site.