在普通窗体中可以通过.PaintPicture实现这个要求，可是MDI窗口没有此方法，怎么才能实现呢？

用PictureBox作背景

同上

picBack是一个PictureBox控件
    picBack.Picture = LoadPicture(strPicPath) '加载图片
   
    picBack.PaintPicture picBack.Picture, 0, 0, picBack.Width, picBack.Height
为何还是没有拉伸效果？？？

用API函数StretchBlt吧，它可以拉伸图片

Private Sub MDIForm_Load()
  LoadNewDoc
End Sub

Private Sub MDIForm_Resize()
Form1.Move 0, 0, Me.ScaleWidth, Me.ScaleHeight
Form1.Image1.Move 0, 0, Me.ScaleWidth, Me.ScaleHeight
End Sub
Private Sub LoadNewDoc()
    Static addform As Long
    Dim frmD As Form2
    addform = addform + 1
    Set frmD = New Form2
    frmD.Caption = "Document " & lDocumentCount
    frmD.Show
End Sub

Private Sub new_Click()
  LoadNewDoc
End Sub