Visual Studio conversion wizard doesn't upgrade VB6 ImageList to the .NET ImageList. You may want to extract the images as files so that you can create a new .NET ImageList with same images. 

    Sub ExtractVB6ImageListImages()
        Dim nCount As Integer
        Dim nIndex As Integer
        Dim sKey As String

        nCount = Me.lstTreeViewIcon.ListImages.Count()
        For nIndex = 1 To nCount
            sKey = Me.lstTreeViewIcon.ListImages(nIndex).Key

            Dim oPictureDisp As stdole.IPictureDisp = Me.lstTreeViewIcon.ListImages(nIndex).ExtractIcon
            Dim oIntPtr As IntPtr = New IntPtr(oPictureDisp.Handle)
            Dim oIcon As Icon
            oIcon = System.Drawing.Icon.FromHandle(oIntPtr)

            Dim oBitmap As Bitmap
            oBitmap = oIcon.ToBitmap()
            oBitmap.Save("C:\Icons\" & sKey & ".jpg")

            '' The following code will get "unknown image type" exception
            'Dim oImage As Image
            'oImage = Support.IPictureDispToImage(oPictureDisp)
            'oImage.Save("C:\Icons\" & nIndex & ".jpg")
        Next

    End Sub