Hi,
It's probably the return URL that you configured in your payment methods under the menu System>Payment methods.
Otherwise, it might come from a customization of the file "after_end" of the view "checkout" via the menu Display>Views.
Or, les likely, a core file of HikaShop being modified by a developer on your end, or a custom HikaShop plugin developed on your end.
You can edit the emails via the menu System>Emails. There, you can change the link to the img tags in the HTML version.
For the invoice you can get in the backend, you can edit the file "invoice" of the view "order" for your backend template via the menu Display>Views. For the invoice your customers can print on the frontend if you've activated that, it's the file "show" of the view "order" for your frontend template.