slimage

It will compress the thumbnails that WordPress generates but will leave the original file intact, so that you can restore your changes if necessary..
That way you have absolute control on how much quality you are willing to sacrifice order to achieve better performance improvements.
Using it in conjuction with the media regenerate command of WP-CLI or with some thumbnail regeneration plugin, you can bulk optimize your photos and even experiment until you find the combination of quality size that suits you..
To use the plugin you need to have jpegoptim and optipng installed on your server and make sure that PHP shell exec function isn't disabled in your php.ini..
The rest of the samples are compressed with 60%, 50%, 40%, 30%, 20% and 10% ...
After you set your desired compression level on the plugin settings and save your changes, you can use the media regenerate command of WP-CLI to regenerate the thumbnails.
If you don't feel comfortable with the command line and prefer to do it with plugin, Force Regenerate Thumbnails and AJAX Thumbnail Rebuild have been tested and confirmed to play nice with Slimage...
If only one of the tools exists, then it will use it to process the specific type of images only ...
Depending on the complexity of the image, though, on some cases the quality loss might be visible to the naked eye so you might need to experiment with the level of quality...
Browse the code, check out SVN repository, or subscribe to the development log by RSS...
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