The previous method didn't work for me so I'm posting an alternative method.

Change:

DIR_WS_ICONS . 'preview.gif', ICON_PREVIEW

To
tep_image(DIR_WS_CATALOG_IMAGES . $products['products_image'], SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT)

Whole section looks like this:



It's not perfect so improve it if you can

More...