What happens if I don't pay $780 demanded by Getty Images for copyright infringement?

If a company owns a copyright in a work and you use that work without permission, you have violated their copyright. Even after the work has been taken down, the offense has been committed. They can file a complaint in court to collect damages from the offense. To collect, they will need to have a registered copyright on the work. I would ask for the registration number of the work, which you can then verify through the US Copyright Office at copyright.gov. If they have registered in advance of your offense, they will have the possibility to sue for statutory damages (which could easily be $780) plus their attorney fees.