I have been noticing this bug. While the recent update renders all profile pics Retina-ready (HiDPI), the Gravatar-based profiles are still blurry on a Retina Display. Here is a screenshot:
Also, if the profile picture is uploaded directly to StackExchange, then it loses alpha channel (transparency), so currently it's impossible to have a picture which has transparency and sharpness everywhere.
Please address this issue.
I just noticed: Meta doesn't support this too.