Get better uploaded quality or save bandwidth: Change the JPEG compression-level of uploaded images and thumbnails.
First, install this plugin and choose your new compression level. Then install the Regenerate Thumbnails plugin, http://wordpress.org/plugins/regenerate-thumbnails/, and run it. All the regenerated thumbnails will be compressed at the compression % from this plugin.
The default image compression setting for JPEGs is 90%.
After you deactivate the custom compression setting will be removed and all new images will use WP's default 90% compression setting. Images uploaded while the plugin was active will stay at their old compression level.
Unfortunately, it doesn't. All the PNG compression tools I've found aren't standard on most servers. You would need root access and a more-complicated plugin to get PNG compression working.