text: Title: [Question] How add an image (from disk) to self-contained-html
  Body: Edit: got ist, need to convert the base64 bin file to string
  Edit2: seem not to be enough :-)
  Hi,
  
  I'm relativ new to python/pytest. 
  
  I want to add some images to the html report directly. As the documentation says, its not (always) possible as path, so I want to add it directly. 
  
  I tried something like this, but it doesn't work :-)
  ```python
  # [...]
  picpath="/path/to/my/pic"
              # add image as b64 to embed it in the html file
              with open(picpath, mode="rb") as pic:
                  b64pic = base64.b64encode(pic.read())
              extra.append(pytest_html.extras.image(b64pic, mime_type='image/png', extension='png'))
              report.extra = extra
  ```
  The report is created and the image is in the html file, but not shown. So I think, there is still an issue with the "b64pic" part.
  
  Can somebody give me a hint?
  
  
  
  Thanks a lot