Image.Save() throws GDI+ exception for image derived from bitmap - vb.net

Question

I have two classes: ChartObject & GrowthChartPane.
ChartObject has a method, GetChart(), that converts a bitmap to a Drawing.Image that is returned.
Public Function GetChart() As Drawing.Image
Dim chartImage As Drawing.Image
Dim gr As Graphics
Dim brush As Brush = Brushes.Blue
If m_bIsBoy Then
brush = Brushes.Red
End If
Using fs As New FileStream(m_sChartImageFileName, FileMode.Open, FileAccess.Read)
chartImage = Bitmap.FromStream(fs)
gr = Graphics.FromImage(chartImage)
End Using
For Each Point As PointObject In m_lstUpperPoints
If Not Point.IsVisible Then Continue For
Dim x As Integer = m_objUpperHorizontalAxis.ValueToPoint(Point.HorizontalValue)
Dim y As Integer = m_objUpperVerticalAxis.ValueToPoint(Point.VerticalValue)
gr.FillEllipse(brush, New RectangleF(x - 4, y - 4, 8, 8))
Next
For Each Point As PointObject In m_lstLowerPoints
If Not Point.IsVisible Then Continue For
Dim x As Integer = m_objLowerHorizontalAxis.ValueToPoint(Point.HorizontalValue)
Dim y As Integer = m_objLowerVerticalAxis.ValueToPoint(Point.VerticalValue)
gr.FillEllipse(brush, New RectangleF(x - 4, y - 4, 8, 8))
Next
DrawHeaderAndTable(gr)
Return chartImage
End Function
GrowthChartPane, when it is loading, makes a call to ChartObject.GetChart() to instantiate a Drawing.Image object. When trying to Save the image that is returned from growth chart, the exception occurs. The method is pretty big, but here is a snippet from the end where the exception is.
Dim fn As String = PediatricGrowthChartsImageHandler.GetPGCImagePath(CurrentPatient.EntityID, m_iChartTypeId)
If Not IsNothing(customDrowingChart) Then
Dim chartImage As Drawing.Image = customDrowingChart.GetChart()
hdnImgChartH.Value = chartImage.Height.ToString 'test
hdnImgChartW.Value = chartImage.Width.ToString 'test
chartImage.Save(fn, System.Drawing.Imaging.ImageFormat.Png)
chartImage.Dispose()
imgChart.ImageUrl = String.Format("PediatricGrowthChartsImageHandler.axd?PatientID={0}&PGCTypeID={1}&rnd={2}", CurrentPatient.EntityID, m_iChartTypeId, New Random().NextDouble().ToString())
Else
Chart1.SaveImage(fn, ChartImageFormat.Png)
End If
End If
If Not IsNothing(DataToBeFilled) Then DataToBeFilled.dispose()
End Sub
I have confirmed that the file does NOT exist, yet exception is still thrown on Image.Save()
I've done some research on this topic & there are so many different solutions out there, but that brought me to a concern about the stream being open.
I've read that the stream needs to be open to Save an image and that you can directly pass a stream into the Save() method, but I 'm curious about how that interaction goes between the two classes.