We wrote a batch jpeg/png/gif compressor that watches folders and auto-compresses new/altered ones. It's web application agnostic, too.
We finally ran it on a substantial dataset and did extensive visual lookover of the result. You need to get to 4x magnification before noticing any difference in before/after, very rare is the image that you notice a difference in at 3x, or a slight shift in tone.
cjpegli, pngquant, and gifsicle are badass. The other part of the secret sauce is conditional rescaling, and not replacing an image unless we save at least 7% bytes.
Next, we will run it through some wordpress sites and see how it compares against the best free plugins.
If we also see such good results in wordpress, then the logical thing is to package it up for open source; too good to not give away!