It is possible to display an image thumbnail in a dialog with something along these lines:
dlg.Control("ImgThumb").label = filespec;
and a control that looks similar to this:
<control halign="left" height="200" image="yes" name="ImgThumb" type="static" width="200" x="10" y="15" />
This technique does not work if filespec points to a video file, rather than an image. However, the video's thumbnail is displayed correctly in a DOpus lister so I am hoping there may be a way to obtain it in a script. I tried using DOpus.LoadImage(filespec) but that generates an error when filespec points to a video.