Do PDF's have functionality to read an image from a url when it's opened? Would be similar to some javascript fetching an image on window load. Not sure if PDF's have that functionality, or can run javascript to mimic that functionality. Saw somewhere that maybe PDF forms could dynamically change an image href.

If so, is that something I can include in creating my document with PrinceXML?

This may be something that can be done with JavaScript inside a PDF file, but we haven't done anything like this ourselves, and there is no feature in Prince for this purpose. I must admit it is an unusual request.