Images are requested by the client in a separate HTTP GET. You'd have to create another Action that takes that separate request and returns the contents of the graphic along with the appropriate MIME type. Strut's "image" tag may be a starting point for you.
As Nathaniel previously said, all you need to do is to code a Struts Action that will take care of the picture 'retrieval', then call this action from the image tag of your JSP.
For exemple, you you might have a DisplayImageAction class with an execute method in it. This action is supposed to return a stream of bytes, so you may set the returning type of the execute method to void instead of the default ActionForward since no forward is involved here.
Code the access of your stored procedure right into the execute method. Basically, get a byte containing your picture, set the ContentType of your response (at least "image"), then send the bytes to your response outputStream (response.getOutputStream).
Once it's done, just call your Action from your JSP with basic HTML tag or Struts IMG tag. Don't forget to send a parameter in your request and use it in the Action to identify the file you need to retrieve :