This usually happens because of image resolution or how the image is being scaled. Here are some steps I would recommend:
1. Check Image Resolution:- Make sure the image is high-resolution. For mobile, it’s best to use at least 1200px wide. If it’s a hero banner, go for 1920px wide to cover most screen sizes.
The image should be in PNG or high-quality JPEG format to maintain clarity.
2. Responsive Image Settings:- Check your theme settings. Some themes compress or resize images for mobile, which can cause blurriness. Look for settings related to image optimization or lazy loading.
3. Use Separate Image for Mobile:
Some Shopify themes allow you to upload a different image for mobile. This lets you optimize the resolution specifically for smaller screens.
4. Test on Multiple Devices:
Use Chrome’s Developer Tools or other responsive design testers to see how the image looks on different screen sizes.