Dim img As System.Drawing.Image
img = System.Drawing.Image.FromFile(Dir)
Dim callback As Image.GetThumbnailImageAbort
 
Dim intWidth As Integer = 100
Dim intHeight As Integer = 100
 
'Richtiges Seitenverhältnis ausrechnen
If img.Width > img.Height Then
    intHeight = img.Height \ img.Width * 100
Else
    intWidth = img.Width \ img.Height * 100
End If
 
'um hier ein richtig dimensioniertes Thumbnail zu erhalten
ImageList.Images.Add(Dir, img.GetThumbnailImage(intWidth, intHeight, callback, _
  IntPtr.Zero))