Elle here from Shopify.
When you upload your product images from Printful, you're given the option to either upload a JPG or PNG mockup. JPG images, while lesser in quality than PNGs, will allow your site to load faster. Here is a helpful video from Printful on mock up images.
It may be also worthwhile to ensure you have your cache and cookies cleared within your browser to ensure you're not experiencing any browser related issues.
Printful's support is also happy to take a look at the uploaded images on your site to ensure there are no issues present. You can contact their support via live chat, email or by phone here.
Hope this is helpful!
|an hour ago|