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.

brokenlibrarian

SimpleJPEGQuality is a plugin that adds a JPEG quality setting to the WordPress media settings page, which is used to determine the image quality of thumbnails, post images, headers, and other images which have been uploaded and then resized by WordPress. It will have no effect on the quality of the original uploaded images, or that of any images that were resized before this plugin was installed.
This plugin will also affect the quality level of JPEGs resized by plugins that use the image functions of WordPress (such as Auto Post Thumbnail). Image plugins that use their own code (such as Lazyest Gallery, Ungallery, and Post Thumbnail Editor) will not be affected.
The plugin has no extra requirements and has been tested with the twentyten through twentyfourteen themes.
Development on this plugin has halted. Other developers should feel free to use the Apache-licensed code for their own projects.
http://brokenlibrarian.org/tinyplugins/ 
brokenlibrarian@gmail.com 
09/28/14
<h3>License</h3>
Copyright 2014 Christian Wagner
Licensed under the Apache License, Version 2.0 (the “License”); 
 you may not use this file except in compliance with the License. 
 You may obtain a copy of the License at
<pre><code> http://www.apache.org/licenses/LICENSE-2.0
</code></pre>
Unless required by applicable law or agreed to in writing, software 
 distributed under the License is distributed on an “AS IS” BASIS, 
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 See the License for the specific language governing permissions and 
 limitations under the License.

SimpleJPEGQuality

SimpleJPEGQuality is a plugin that adds a JPEG quality setting to the WordPress media settings page, which is used to determine the image quality of t …

Image Quality vs SimpleJPEGQuality