You can not show new picture in Rt_Picture field in UI without saving (in background) and re-opening(in UI) the document. Make sure to not save in UI after saving in background as it will rewrite your background changes.
From help file: Modifications made to rich-text items on the back-end document do not appear on the current document until it is closed and reopened.
You should however be able to use Import function without re-opening the document:
Dim workspace As New NotesUIWorkspace
Dim uidoc As NotesUIDocument
Call uidoc.Import("GIF Image", "c:\tmp\mypic.gif")