It would be great if you implemented a crop/preview feature, so the user could line things up properly instead of having to manually loop through editing/submiting until a decent result is achieved.
You asked for it HN, so we'll show you a preview of the upcoming UI: http://fromjeffrey.com/pc/
Features: - Live Preview - Translate (Move) - Rotate - Resize
This is 110% an alpha build and the UI def. has to be more intuitive (like rotate is based on mouse drag in X-axis, but how does the user know that??)
Uses processingjs to do the heavy lifting with PHP backend - still getting out of memory errors with large pictures, so try to start with something reasonable (IE: not 5 megapixels dimensions 2000px and smaller recommended)
Have fun and please leave comments! =D