OK here is the easier way. It's much faster to implement, but it assumes that all files (normal, featured & listed) are all in one directory, instead the above way allows you to have they sorted out in different directories. So here we go.
Create a file called /product_thumbnail_feat.tpl
Code:
{if $config.Appearance.show_thumbnails eq "Y"}[img]{if $tmbn_url}{$tmbn_url|replace:[/img]{/if}
Now all you have to do is look through /customer/main/products_t.tpl and replace the call for product_thumbnail.tpl to our newly created file. That's it! No extra functions to create or anything. I'm still using the original way because I have a lot of products and it's easier to have pics in different directories.
Again this will show the featured product images from example.jpg to example_f.jpg. Also it's best to have all files in one format, but if not creating a function that checks if a file is .gif or .jpg wouldn't be hard.