Load image from URL on file open, then embed image

Question

I am trying to create a PDF form with an image field that is empty/blank by default. When the PDF is opened, I want it to load the image at C:\picture.jpg and embed that image in that document.
I need it to load on file open because C:\picture.jpg will be different each time the file is opened. Currently, if I set the image URL and embed it, it uses the image at the first time it's embedded.

Answer 1

Okay, I created userInfo.js and placed it in C:\Program Files (x86)\Adobe\Acrobat 10.0\Acrobat\Javascripts. That file (adapted from How do I change an image in a button automatically without changing its location?):
myIcon = app.trustedFunction( function (){app.beginPriv();
this.getField("userInfo").buttonImportIcon("C:\\picture.jpg")
app.endPriv();})
I then added event.target.myIcon(event.target) to topmostSubform.Page1::initialize.
I get the following error when opening my pdf:
TypeError: this.getField("userInfo") is null
2:XFA:topmostSubform[0]:Page1[0]:initialize
I know the field is null. I want it to be null until I open the pdf. Is there another reason that error would be appearing?