Basically you want to insert the standard code for displaying thumbnails into your template.
Something like this:
Code:
{include file="product_thumbnail.tpl" productid=$product_links[cat_num].productid image_x=60}
Put that next to the description, or make a table to display it how you want.
For Upselling products this would be in:
skin1/modules/Upselling_products/related_products.tpl