probably the best way would be to look at the nifty video template (or possibly combine the nifty video template with the slides template )

there are 2 fields that allow you upload physical files, 1) file and 2) image. In slides, the upload form only has a [TAG] for the image and the template displays a thumbnail of the image and a full sized image when you click on.  In the video template, there are [TAG]s that allow you to upload both a file and an image and the template displays a thumbnail of the image, and plays the video when the image is clicked on.

You'd also have to modify the template to provide a method to display the fullsized image. you could add a hyperlink under the thumbnail that would display the fullsized image when clicked on, since clicking on the thumbnail plays the video.  If you look at the nifty video template, you'll see the OBJECT tag that embeds the media player. To add a hyperlink under the thumbnail that will display the full sized image add the following in the template.html file where you would like the hyperlink to display.