Compressed images will sometimes experience significant quality loss. This can be related to image dimensions, export settings, or the image compression plugin.
Change compression level
The most immediate fix would be the revert that one image (or re-compress it as “Glossy” instead of “Lossy”). This will significantly increase the file size, but will recover the quality.
Use retina images
We can also try creating retina thumbnails for your site. In some cases, these will be used automatically and significantly improve quality on high-resolution devices.
If those thumbnails are not used automatically (as will be the case for some placements), a more time-intensive fix would be to implement a feature known as “srcset” on your site.
Known issues
Compressed multiple times
Some clients have had weird situations where images will be compressed multiple times, causing significant quality loss. To check if this is the case with you, revert the image to the original, then recompress “lossy” and compare quality again.
Image settings
In some cases, the image itself is the issue. Some colors or export settings seem to have less appealing compression. This would be a good question for your image compression plugin support.
Text on images
Usually, images with text are the easiest place to spot compression degradation. One option here is to skip compression on all images with a certain phrase in their filename, such as file-name-dontcompress.jpg. This phrase can be set in ShortPixel’s settings.