If you're experiencing blurriness when uploading high-quality images to your website, it could be because the image size selected in the Attachment Display Settings is smaller. To resolve this, ensure the 'Full Size' option from the list is selected when uploading your images, as seen below:
If you selected "Full Size" and it's still blurry, ensure the uploaded image is large enough. Please take a look at the Image Format And Size Guide help file.
Note that if you use the image as a background image or use an aspect ratio, it will often be re-sized to fill the area.