I am on the latest beta of Anki on windows, and have been having this issue for months with many prior versions. Whenever I copy and image directly from a browser, and paste it into Anki, the picture in Anki is much worse in quality compared to the original. If I download the picture and add it manually into Anki, then the quality is the same as the original.
I currently have the “Paste clipboard images as PNG” option enabled.
As you can see in the picture below, the difference is notable between the original on the right and the copied version on the left:
A common cause for images to be of low quality is when it is enlarged without proper tools. Increasing the size of the image can often result in the photo losing its quality and resolution to drop significantly.