Colour quality reduction in photobucket.

I've recently moved from a windows laptop to a MacBook and from PS7 to PSCS2. I save my icons as .pngs and they didn't lose any colour depth when I uploaded them to photobucket, however since I've moved I've noticed a slight reduction in colour quality when I upload to photobucket. The colours lose their depth and appear less rich. It's not seriously noticeable but is there any reason for this or is it just something I'm going to have to get used to?

I have looked in the memories but I could really only find things relating to saving for web to preserve transparencies, so if I have missed something, I'm sorry.