The profile image shown by Discourse is much darker than the one delivered by Gravatar or shown by e.g. StackOverflow. I suspect it has something to do with the processing required to turn it into a circular icon.
Just a shot in the dark, take a look at http://stackoverflow.com/questions/15399653/image-conversion-to-grayscale-using-imagemagick-is-very-dark
This has come up on meta from time to time
The weird thing is though that you are using gravatar here, there shouldn't even be a local image.
Definitely a difference though.