I changed my user image after the switch to the new Stack Exchange header, using a png with a transparent background.
Upon upload, I noticed that due to the fact that imgur does not support transparency in re-sized images, user images with transparent backgrounds (uploaded directly to SE) in the black bar will always have a white square around them:
See this question:
The workaround is to use Gravatar, but the fact that you cannot simply upload a transparent png to Stack Exchange bothers me...
Maybe without support for resizing, Stack Exchange needs to have a small, black-background user image stored for the purpose of using in the menu bar? Or is my obsessive-compulsive personality just showing too much?