> For the PNGs, I had used 4 different optimizers (PNGCrush, OptiPNG, AdvPNG, and PNGOut), but almost all images saw size reductions in lossless mode ranging from under 1% to over 50%

I seriously doubt you can squeeze more than 1-2% after properly running PNGOut on an image.

This service only makes sense if you have huge images that you want to optimize. $19 for 1000 8-megabyte images is a pretty good deal. That's a rare case though. If you have tiny image, just run PNGOut on them for free.