Can vouch for Kraken.io -- I've just begun using it to optimize ~50k+ interior architectural images so fidelity is important to me and my users. I'm about a third of the way done with the project and am saving just about 40% in size on average.
I'm still thinking on the best way to implement it. Right now you can mount an FTP/SFTP folder and process from there. But I might try to bypass the mounting part. Server version is also in the works.