I’ve just pushed an update to the community site which fixes an issue where transparency was being removed when uploading transparent PNGs.
Unfortunately you’ll have to re-upload all affected icons — note that this doesn’t affect your screenshots and that not all icons have had transparency removed.
All you need to do is go in and re-upload your transparent icon.
This is especially important for any addons you are promoting, as we have an updated design that works best with transparent icons.
Give me a shout if you need any help / more info. Sorry for any inconvenience caused!