Javascript gif images question

Hi guys.

I've 2 images in a normal page, each image has a transparent Bacground.

The first Image.
http://img32.picoodle.com/img/img32/...2m_b578e2f.gif

The Second Image.
http://img32.picoodle.com/img/img32/...1m_038aa51.gif

And this is how they look like in the page.
http://img37.picoodle.com/img/img37/...3m_9d67721.gif


the user can drag both of the images anywhere in the page.
I've problem in calling an event when both images cross each other because the images border are irregular shape.


I tried to do that by checking the position of each image, but that doesn't work accurately, because the code will call the event in case of the two images cross in the transparent background too..

I want a code that call the event when the two images cross just in the real border.

http://img37.picoodle.com/img/img37/...2m_00ff040.gif