Shopify has changed how they store images on stores, images are now stored as WebP Images. 
Below is a link to their Changelog where they posted the change on the 17th of July:
https://changelog.shopify.com/posts/online-stores-automatically-serve-webp-images
If your having a problem viewing them, the following link will add an extension to windows computers to allow them to be easily viewable:
https://techdows.com/2011/04/viewing-webp-images-on-windows.html
You can also replace any upload fields with a Filestack uploader that would save the file outside of Shopify so it wouldn't be converted. Click here if you want to use Filestack instead!