A. You only need the 2nd line (Set oDoc = GetObject("C:\Validation\FirstDoc.rtf", "Word.Document")) to make it work.
B. The link you've provided refers to a web page, not a word document.
The actual object model you want is:
Word.Document Com Object. And indeed this object model doesn't have the property you're trying to access.
In Word, pictures are considered to be shapes. So you actually need to access the .Shapes and .InlineShapes collections of the document. In your script, this will translate to accessing the oDoc.Shapes and oDoc.InlineShapes collections respectively.
You can read about the different properties and methods for these collections
in this MSDN article (It's for word 2003, but these collections haven't changed much since Office 2000). I hope this is enough to get you started (If not, you can always post new questions under this thread).
Enjoy

Maybe I'll write an article on the matter in the coming weeks.