How can I remove the product thumbnails below the main product image?

You can hide the theme's thumbnails by following these steps:

1. Sign into your Shopify admin
2. Go to 'Online Store' > 'Theme'
3. Click 'Actions' > 'Edit Code'
   
4. Scroll to the 'Assets' folder and select the 'theme-custom.scss.liquid' file
5. Paste in the following line of code:
   

   .secondary-images { display: none; }