Gagan Deep Singh

Sometimes you may want the images on your blog/site to be pixel perfect, while sometimes you may want them to be compressed to save space on the server.
This plugin allows you to select the quality of the thumbnail images generated by WordPress, as easy as that. Just go to Media Settings page and modify the Image Quality setting.
Plugin only works for new uploads, but in case you want to modify the thumbnails generated earlier, you can either use
<a href="https://wordpress.org/plugins/regenerate-thumbnails/" rel="ugc">Regenerate Thumbnails</a>
or if you are familiar with WP-CLI, then you can use this command to regenerate
<pre><code>wp media regenerate
</code></pre>
P.S. It does not change the quality of the original image that you upload, 
but only changes the quality of the thumbnails WordPress generates from that image.

Image Quality

Lets you adjust the quality of image thumbnails that WordPress generates.

Yonatan Ganot

Disable JPEG Compression

Disable the JPEG compression in WordPress, which enabled by default.

Image Quality vs Disable JPEG Compression