Hey @tinypng,
thank you for your answer.
These weren’t theme specific images, these were content images from /wp-content/uploads
We already replaced them manually via ftp so there is no point in checking the site now.
Glad to know that it’s not you who made a bad change in a plugin, that it’s Google who changed their algorithms.
My suggestion is, that it would be cool if you could add a lossy compression option though, to satisfy the big G for the time being.