Watermark images using PIL

[Thomas]

I've searched google for this topic and found a few examples, but I
need an example of how to create a semi-transparent, non-intrusive
watermark for images using PIL, something that takes into account the
background of the image when choosing color for the text. 

Making a huge white box or just putting a single-color text across the
entire image looks just bad, so if there is a way to create a more
professional looking watermark I'd be happy for any hint or tip you
might have to offer.

Best regards,
Thomas