In the original version of the mod I dealt with this by having an additional thumbnail and matching medium image for the original image - ie the original image is represented twice, and in four images, but the customer is unaware of this.
Do you have the latest version of the mod working otherwise? I don't yet - I have the same problem as in this thread
http://www.zen-cart.com/forum/showthread.php?t=51419&highlight=swap+images
which is that the thumbnails are not calling the _MED version of the image, they are only calling themselves. You can work around this by simply installing a medium size image as a thumbnail but this is a big waste of bandwidth and could be very off-putting for any customer on a slow connection, especially if you have a few images per product.
If you read this Dr Byte, are you aware of this problem?