It is possible to upload the images from the browser, without using Git.
Create a new GitHub issue and drag the image into the textbox.
GitHub will upload the image and create a link like this one:
https://f.cloud.github.com/assets/19977/1656110/a3f8b280-5b6d-11e3-818d-c06ab05bd613.jpg
Source: GitHub’s Phil Haack