How you add an image to a Leanpub book depends on your writing mode!
If you're writing in our In-Browser Text Editor, here are the instructions:
If you're writing in the GitHub or Dropbox writing mode, here are the instructions:
Finally, if you'd like to insert an inline image, here are the instructions:
http://help.leanpub.com/en/articles/3919168-how-do-i-insert-an-inline-image-in-markua