Here is the image file: https://gist.github.com/mbostock/5503544#file-thumbnail-png
I tried to drag-and-drop the image file in the edit mode of a gist. That didn\'t
I was looking for answer for the same question, here is what I just found:
You can clone your gist as a git repository and add your picture, then push to origin. See loading-thumbnail-into-gist-for.
There is a script written to do this: hecticjeff/gist-img.
I have not tried the above solutions yet, but pretty sure they should work. I did try using defunkt/gist to update my gist with a png but ended up displaying a binary file in my gist.