My name is Alex, I'm a guru here at Shopify!
Thanks for reaching out today! Simple was designed to be a mobile-first, responsive design - this is already built into the theme's code!
It can happen that large images can be cropped for better display on certain window sizes, or for the mobile view - this is because in certain cases, showing the entire image would shrink it to the point of illegibility. There's more on best practices for slideshows and full-width images in our guide right over here. For Simple, we recommend image sizes of 350 x 1024px (or an aspect ratio of 1:3) - if you need to edit those photos, you can use a photo resizing service such as Picresize if you don't have a program on your computer that can help!
Hope that's helped! Please feel free to let me know, and you can always give us a call or start a live chat at any time. We're open 24/7 for your convenience and always happy to assist!
Alex | Shopify Guru