Ignore transparent areas with watermark

[Liquix]

Hi,
Im making som new scripts for a phpBB forum I own. I got some problems with a detail with my watermarks.

Here is a image automaticly watermarked by my script:
http://images.habbopalace.no/?id=efdc2b22d592f28ca90cdfc6664021d7

This is the watermark Im using:
http://images.habbopalace.no/?id=watermark

Im setting that purple color as transparent, resizing and using a 20%  opacity when adding it on the image. As you can see on the test image the watermark is added over transparent areas to, I want it to ignore those areas only be placed where the actual image is. Is this possible?