For some reason, WordPress does not seem to give you an option to increase or decrease the JPG compression ratio in the media settings. This means your scaled images are stuck at the default compression ratio. The default is 90%. This is quite high and when used with high resolution, high-quality images, this is not going to be noticeable. If you are finding your images are too slow to load, 60% is a good balance of quality verse speed, but it will be a noticeable quality difference. Sometimes though, even 90% is really not good enough and you need to increase the quality, or your images are just too slow to load and you need to decrease it.
At this time there are only two ways I know of to increase or decrease the quality.
1. Use a plugin
If you are not confident editing your theme, the easiest way is to use a plugin like this one. It has both free and premium version that allow you to configure your compression settings and optimise your existing and new images.
2. Adjust it in your theme functions.php file
If you are confident editing your theme then it is possible to set the quality in your theme’s functions.php file. It’s just a matter of adding this snippet in (or if it already exists, editing it):
add_filter(‘jpeg_quality’, function($arg){return 100;});
This snippet will set the quality to 100%. If you would like to go with a lower quality setting, just change the number from 100 to the percentage you would like.
Just remember: higher quality images = slower loading time. Lower quality images = faster loading time.
