There is a glitch with 255x255 icons that I was hoping to get around to fixing by the next version but 256x256 I thought worked ok. I've tested it with certain 256x256 icons and they load ok. Can you upload some of the icons that don't work on the web and send me a link to them? Then maybe I could see what's happening with them.
I can't find the file right now, but IIRC it was one that used the Vista PNG compression for the 256 size. If I find it again, I'll make sure I let you know.