All I can say is...why are you using a textbox in the footer?????
Nevertheless, have you tried recording a macro? It works. I had it find a graphic (in a textbox) in the main stroy, copy it, go into header/footer, find a graphic in a textbox (in the footer story), delete it, then paste the one I got from the main doc.
However, the botton line is, no you can NOT edit the existing textbox contents. While you can select it, you can not get the insertion point into the text itself. But you could delete in and replace it with another one.
An alternative, if it is really a graphic you want in the footer, is to use InLineShapes to put a graphic file.
Selection.InlineShapes.AddOLEObject ClassType:="Paint.Picture", FileName:= _
"D:\whatever\blah.bmp", LinkToFile:=False, DisplayAsIcon:=False.
If you put DisplayAsIcon as True, double clicking the icon with open your default image viewer for the filetype.
What are you doing this for again?
Gerry