PNG alpha in particular was a nightmare for me. After that I knew to look out for the pain caused by occasionally-supported features in images.
http://caniuse.com/#feat=png-alpha
PNG alpha transparency was a problem with the decade old IE6. And even then IE6 worked fine with PNG8 images that had alpha transparency. IE7 and IE8 required a 1 line style attribute to work 100% with PNG + alpha, and IE9 did away with that need altogether.
They have a service that tries to perform loss-free optimization of served images. It's easy to disable, and should be disabled if you either already perform similar optimizations (optipng / pngquant, jpegtran, etc) or have images with annotations, color profiles, etc.
Has that changed?
Though I remember that sometimes previews (or was it layers) saved in the .tiff were making it 10x, 20x (not kidding) times bigger, so a special "save" plugin was done to filter these out, and on regular someone wrote a script to check for such.