add image thumbnails in order list back end?

The images data is not loaded there.
So you can't just add a line of code to have the images displayed. You would first have to have a query to the database in order to load the images, add the images for each of the product objects and then display them.
You can do that in the file "show" of the view "order" of the back end via the menu Display->Views, but it will be difficult if you're not a developer...

It should be something like that:

SELECT * FROM #__hikashop_file WHERE file_ref_id =XXX AND file_type='product' ORDER BY file_ref_id ASC, file_ordering ASC, file_id ASC

That would return you all the images data of the product with the id XXX.
Then, you would have to load the image helper: $imageHelper = hikashop_get('helper.image');
And finally use it to display the image based on the data from the query, as we do in the file "show_block_img" that you can see via the menu Display->Views.

Note that this is a development project which is outside the support contract included in the Business edition. I already gave you more than enough information so that a developer could do it by himself, but if you want us to do it for you, then we would ask that you request a quote for that via our contact form:
www.hikashop.com/en/contact-us.html