In the past few months, we have covered a couple of ways you can go about changing the JPEG compression level on your site. Not everyone wants to make small changes to their code to accomplish this task. The good news is you can rely on these plugins to control JPEG quality on your site:
SimpleJPEGQuality: a simple tool that adds JPEG quality settings to the WordPress media settings page. Lets you change JPEG quality easily.
WP Resized Image Quality: lets you change the JPEG compression level of uploaded images and thumbnails from the Media Settings page.
It is also possible to accomplish this by writing your own plugin. Have you found better plugins to control image quality? Please share them here.