If you don't already have an existing gist, you'll need to create one before proceeding.
git clone https://gist.github.com/<hash>.git # via https
git clone [email protected]:<hash>.git # via ssh
Make sure the images are at the root of the gist and not in a subdirectory.
git add <some-image>.<jpg|png|gif|etc.>
git commit -m 'Add Image(s)'
git push origin master