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Yes, this is correct. The product images are files, not database records. I don't remember where VirtueMart stores the product images. I think it's in the shop_image directory somewhere inside components/com_virtuemart. You will have to manually transfer those files as well.

In Akeeba Backup's Control Panel page you can find the "Database Table Exclusion" button. Click on it. You can now exclude all database tables except those containing the product details.

Please note that a database only backup is what it says: it can only back up database content. Since files are not database content, it is impossible to include files with it.

I think that what you need is to have two backups running:
1. A database only backup which has all tables excluded except your products table; and
2. A files only backup which has all folders and files excluded except your product images

What you are trying to do is not a simple procedure. You are trying to synchronise two or three sites. It is a very involving process that requires a great deal of technical skill. You have to know where the data you want to synchronise is, back up only this data and restore it to the other sites. It sounds difficult because it is difficult.