Always use a lossless format, preferred is PNG. Never use JPG for screenshots or computer graphics. JPG is lossy and creates coding artefacts which disturb the legibility of sharp lines and text characters.
Upload your screenshot to imgurhttp://imgur.com (free; like "PasteBin" or "YouTube" but for images).
Remember the link (delete hash) which is presented to delete your image (the Url is only once presented on the screen immediately after uploading. Save that link for later use if you need it). BTW, Greenshot saves the delete hashes in a logfile of uploads.
Add here on GitHub a tag with the link to your image <img src=http://i.imgurl/....>.