Dragging Issue

This seems to be a problematic day. I have built an example file to demonstrate the following problem:

I have externally loaded an image onto a button (to act as an image button). I have made the button draggable and want it to stay at a certain point when released into a drop area.

THE PROBLEM:
If the user clicks on the image contained within the button, it won’t work. If the user clicks on part of the button not covered by the images, it works. I know this is probably because the overlapping image is blocking my event handler for the button - but it still drags which is most strange. Can anyone help??

All i need is for the button to stay within the box shown in the file when it is dropped within it - but no matter where you click on the button.

The image is represented by a small turquoise blue circle and the button is the blue polygon.

Many thanks