This code samples below will let you add an image to the RichTextBox. The first code uses the OLEObject and the second using the ClipBoard.
 

Private Sub Command1_Click() Dim sfile As String
sfile =  "c:\download\new.bmp"
rtf1.OLEObjects.Add  , , sfile

End Sub 

'Can be used with the PictureBox or Image Control 
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long,_
ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long


Private Const WM_PASTE = &H302

Private Sub Form_Load()
'Load your chosen graphics file into a picturebox or Image control...
Image1.Picture = LoadPicture("g:\oscar.bmp")
'hide it from the user
Image1.Visible = False

End Sub

Private Sub Command1_Click()
    ' Clear the clipboard
    Clipboard.Clear
    ' Copy Picture into the clipboard.
    Clipboard.SetData Image1.Picture

   ' "Paste" the picture into the RichTextBox,
   ' using the SendMessage API function
    SendMessage RichTextBox1.hwnd, WM_PASTE, 0, 0
End Sub