The user would be unlicensed, and in lieu of the vendor resolving this then the user would need to purchase licences to continue using the software legally (ie if a vendor gives you a pirate version of photoshop, you can’t just use it forever just because someone sold it to you).

There are usually clauses in enterprise software agreements that attribute liability for unlicenced components to the vendor for this reason. But ultimately if there isn’t a contract or the vendor vanishes, the user will need to go get a licence.

If you want to test the theory, I’ll send you a few images to put on your website, and when you get a claim through from the copyright owner you can try to argue that I sent it across without a copyright notice so I am liable ;)

No, but the difference is the users of a product are typically not making and distributing copies. That’s not the case if you use someone else’s code in your project.