This sprite has a small script that differentiates between when it is just clicked, and when it is dragged and then dropped. Replace the "think" blocks with whatever you need it to do. The script and variables are self contained in the sprite, so you can put it in your backpack and easily add it to your own project.
Starting point from the wiki: http://wiki.scratch.mit.edu/wiki/Draggable_Sprite_Feature I needed a script that differentiates between click and drag/drop in one of my projects (http://scratch.mit.edu/projects/20125138). The script worked out pretty well, so I thought I'd share it in a way that is easy to copy.