Property releases are similar to model releases.

It would be very helpful if they did, but for a variety of reasons they do not.

Thousands of bills for retrospective licenses have been sent out by both Getty and Corbis, relating to images allegedly used without license by websites.

For an employee carrying out photography in the course of theirr employment, copyright of the work will belong to the employer unless agreed otherwise.

Model releases have little or no relevance to copyright. There is no copyright in the human face or form, so no claim can arise.

Identifying the copyright holder may be straightforward or require some detective work.

In general no. But there are some fair use exceptions. For instance you may reproduce a copyright image for the purposes of criticism or review.

Using an image that is copyright requires permission of the copyright holder. If you do not obtain permission, your use constitutes an infringement. The copyright holder can then sue you for damages.

Yes, absolutely. The web is no different from any other publishing medium.