That seems reasonable to me, but it seems that the Original Poster is
having trouble selecting all the pictures. If this doesn't work, then a
simple VBA script would probably do the trick:

Sub RemoveAllPics()
   Dim oSld As Slide
   Dim oShp As Shape
   
   For Each oSld In activepresenation.Slides
       For Each oShp In oSld.Shapes
           If oShp.Type = msoPicture Then
               oShp.Delete
           End If
       Next oShp
   Next oSld
End Sub

This should delete all the pictures in a presentation if they are really
pictures and not grouped. It will only work for your purpose if there are
not other pictures in the presentation (ones that you want to keep). If
there are other pictures, then you'll need some more conditions that
distinguish the other pictures from the ones you really want to delete
(possibly based on size or location).

If you try this, I strongly recommend trying it on a COPY of your
presentation in case it doesn't do quite what you want.

--David