You can only really do that with javascript (display the image without reloading the page). You’d probably need to use an ajax call triggered by your forms submit button, and after the image is uploaded send back the img src to the success event of the ajax call, which then inserts it into an image holder on the page which will cause the image to load in the users browser.
I don’t use mootools anymore, so I can’t really help you with that part but I’m sure there are a million examples on the net.