Take a look at www.reportlab.org. The ReportLab library includes a graphics module that might well do what you need. I'm not sure at present if it allows one to set alpha-channels to achieve transparency.
Also, if you have access to a mac running OS X 10.4, the Automator application has a prebuilt action that applies a watermark to pdfs. Transparency, size and placement are all editable. Obviously, no use if you want to do this with Python, but it might suit your needs. Rob C -- http://mail.python.org/mailman/listinfo/python-list
