preview an image before uploading it, how?

I made an upload form (in php) that upload some data to a database and a
picture to the web space. everything works fine, but I would like to add the
ability to preview the picture/image before uploading it.
Any ideas? (either automatically or by pressing the a Preview button)
I heard something about java, that could help, is there anybody that can
help about that, or a tutorial?

I have very little knowledge of java if at all. so bare with me. :)
Jim
thanks