You do not have to register your images to own the copyright. The Copyright Act of 1978 makes it clear that when you create it, you own it. But, if you do not register your images and someone infringes on them, you are limited in the amount of damages you can seek in a court of law. Willful infringement can carry minimum damages of about $125,000 for each infringement if you have gone through with the Copyright registration prior to the infringement.
The Copyright office has made registration easier with online tools and so on. It is not the simplest thing in the world but it is also not the hardest. Fees vary depending on what and how you register ( published or unpublished). You can register a large quantities of images that where all shot in the same year if they are still unpublished for around $35.
I would suggest to all who have images that they are really trilled with and that might be posting to this site or others to register the images before publishing them. If you have the occasion to sell any images, make sure Rights and Usage are spelled out on your invoice and make sure you register them.