JSP just produces HTML that is sent to the browser. So you need to use HTML mechanisms, right? Use an <img> tag just as in other HTML pages.
The src attribute of the tag can reference a servlet that returns the image data, along with setting the appropriate response headers. You'll need to find out how to generate a supported image type (gif, jpg, png, etc) from the java.awt.Image instance.
Saving the image to a file in a publicly accessible directory is one way to achieve this.
But ImageIO can also write the image contents into a byte, and then you could use a servlet (which would be referenced in the "src" attribute of an HTML img tag) to stream the image to the browser. That way you can avoid file I/O.