ActiveSheet.OLEObjects("CmdUndoComboBoxes").Picture = ThisWorkbook.Sheets("BASE").ImgButtonMini.Picture.

Sub Test()
    Dim oldButton As CommandButton
    Set oldButton = Sheet1.CommandButton1

    Dim newButton As CommandButton
    Set newButton = Sheet1.CommandButton2

    newButton.Picture = oldButton.Picture
End Sub

ActiveSheet.YourButtonName.Picture = ThisWorkbook.Sheets("BASE").ImgButtonMini.Picture