Using OmniGraffle to Fill Out a Form

I'm trying to use OmniGraffle 5.1 to fill out a form that is in a PDF  
file.  I've done this before, but I'm having a problem this time.   
Here is what I have done.

I dragged the PDF form into the OmniGraffle window and adjusted the  
PDF's (x,y) coordinates so that it is in the upper left corner of the  
canvas.

I then use the rectangle selection tool to create a rectangle that  
fits the area I need to fill in.  The problem is that there is an  
image in the rectangle background.  When I create the rectangle it  
looks like it has a blotchy background instead of white.  If I make  
the rectangle large, I can see that it is a PDF form that I filled in  
several months ago!  If I open the Syle: image inspector, there is no  
image set for the shape.  The "Remove Image" menu item is not available.

How do I get rid of this image?

Is there a better way to go about this with OmniGraffle?

Rick