Drag ad drop images to make a customised gallery
This is actually a nice idea but probably not so easy to implement. 

jQuery UI could help you with the drag and drop features. http://jqueryui.com/demos/droppable/
Drupalgardens does some galerie configuration through drag n drop - so there you may be able to get some inspirations on this. Especially since you can export the site and then look how exacly they do it. http://drupalgardens.com