This process requires that you:
- Add fields to each post type so you have a place to paste in image URLs
- Change content templates so the code knows how to handle those URLs
- Manually edit each post
This will take some effort on your part but that was going to be the case no matter what
approach you used.
Go slow, back up your work, follow the instructions carefully and you can get those images back on your site.
If you have ANY concerns about this approach or any other your BEST source of information will be the Amazon Associates Help Desk so don't forget to contact them with questions if you don't have API access but want to use image URLs on your site.
Thanks!