Runtime Error 481 Invalid Picture

Alter the code slightly.

Private Sub UpdateChart(ChartNum As Integer)
  Dim currentchart As Chart, Fname As String
    Set currentchart = Sheets("Portfolio").ChartObjects(ChartNum).Chart
    currentchart.Parent.Width = 400
    currentchart.Parent.Height = 200

'   Save chart as GIF
    Fname = ThisWorkbook.Path & Application.PathSeparator & "temp.gif"
    currentchart.Export Filename:=Fname, FilterName:="GIF"

stop

'   Show the chart
    Image1.Picture = LoadPicture(Fname)
End Sub

The stop command will pause the running program and take you to the vbe.
Use an file explorer window to check the file size of the exported file.

then use F5 to continue code running