AFAIK, a product such as SuperContainer is the only way to get data into a container via IWP. Container insert simply isn't supported. Since SuperContainer uses a web viewer, it bypasses this restriction.
Another option is to program your own CWP page that inserts the image into the container via PHP. Andy Frazier has developed a nice demo to show this technique at http://reliablesoftworks.info/FMFilesDemo.html.