I added a product to my site http://www.Artofspreadsheet.Com, but I can't see its thumbnail under the "New Additions" section of the home page or under the Products page. Also when I log into my site and navigate to Market Theme > Product Category Images, I get this message:

Product Category either not defined, or it doesn't have any child categories to add images to. 
Please visit the theme options page and select the parent product category.

How do I fix that?

Also, how do I add the product thumbnail that will appear under the store page?

Thank You.