We force product images to be JPG for smaller file sizes. Every little bit toward page performance helps. For most people, this will be an invisible feature that is helpful to speed up their page load speeds.
Of course imagery is some of the most important features of a business and some people will want more fine-tuned control over their product images since Shopify's image compression can be a bit aggressive at times (especially if there is text on a JPG image). PNG format images typically have less compression applied to them which can look higher quality but also come with a — sometimes significantly — larger file size.
To disable this feature you need to make two small changes in your code editor.
First, open up your theme code editor by navigating to /admin/themes, clicking on your copy of your Streamline, and then “Edit code”.
You'll want to open two files, both in your snippets folder:
product-grid-item.liquid
product-images.liquid
Near the top of each file you'll see this line of code:
{%- assign force_jpg = true -%}Change true to false and you are all set. Your images will now be loaded in the format you uploaded.