Sub Demo()
Application.ScreenUpdating = False
Dim i As Long
With ActiveDocument.Range
  With .Find
    .ClearFormatting
    .Replacement.ClearFormatting
    .Text = "<img id=" & Chr(34) & "img001" & Chr(34) & " src=" & Chr(34) & Chr(34) & "/>"
    .Replacement.Text = ""
    .Forward = True
    .Wrap = wdFindStop
    .Format = False
    .MatchCase = False
    .MatchWholeWord = False
    .MatchWildcards = False
    .MatchSoundsLike = False
    .MatchAllWordForms = False
    .Execute
  End With
  Do While .Find.Found
    i = i + 1
    .Start = .Start + 12
    .End = .Start + 3
    .Text = Format(i, "000")
    .Collapse wdCollapseEnd
    .Find.Execute
  Loop
End With
Application.ScreenUpdating = True
MsgBox i & " tags found."
End Sub