When I drag an image that may have been optimized elsewhere (like using Acorn, Photoshop, or ImageMin) Squash says the image can’t be further optimized. That’s OK, no problem. Except that now the image on disk is much larger than before!
For example, I dragged a ~5mb JPG image in. Squash said it can’t be optimized further. Now the image is over 7mb!
Squash should leave the image alone if it can’t help.