You can now attach files, including images, to markdown files while you're editing them in the web. This works just like file attachments in issues and pull requests and supports the same file types. Just drag and drag, click and select, or paste.
Note: If you add an image to a markdown file, anyone can view the anonymized image URL without authentication, even if the markdown file is in a private repository. To keep images private, serve them from a private network or server that requires authentication. For more information on anonymized URLs see "About anonymized image URLs".