I've tried simple image gallery pro and about 8 other image pop-up plug-ins, but none of them could do a single image pop-up. They would only do a gallery of every image in a directory. This meant that it you wanted to place one single image in an article, you had to go and create a directory for it. Finally I can place a single image in an article, and keep all of my images in one, easy to manage directory.
One thing to be aware of, if you're using TinyMCE as your editor, you have to go to Extensions > Plug-in Manager > Editor TinyMCE - and in the first drop down box choose: Extended. This will allow the proper tabs to show up in the image editor when you add an image using TinyMCE while editing an article.
Thanks for the great extension!