Message Input Method : You can use Joomla Articles or Manual Textarea to add contents into the popup.
HTML or Plain Text - For HTML or plain text you can use both the methods. HTML or Text Input or Joomla Articles.
Image - For image you can use the following HTML text <img src="/YOUR_Path"/> and add it into the HTML or Text Input (Message Above or Message Below) or directly insert the image into an article and select and article in the module parameters.
