As I understand it:

In the US, an image is copyrighted when taken. However, unless the image is registered with the Copyrighted Office (it can be done in bulk), you will be limited to the 'fair use' value of the image in any legal action. This means someone can use your image and only pay you what they would have with your permission. If the image is copyrighted, you qualify for triple damages, making if far less attractive to use your image without your permission.

You may have given up your copyright rights if you have a 'work for hire' agreement with your employer.