@Deco - Isn't putting my name on the photo enough? Does the copyright symbol need to be there? I obviously wouldn't provide RAW, but I would provide the largest format I could create in JPG. That gives people more options to print bigger images. How does one register with the copyright office? I assume it is a one time registration (e.g. so they have my name) and I just apply my name to all of my photos? If you have a link to good information I would be happy to read that (unless your answer is short and you don't mind taking the time to post)

Re the © symbol on the photo - yes.......and no...... You don't actually need to do anything to protect your work, but there are many out there who think a) images are free and b) they found it so can use it or c) even if they want to do the right thing, can't find you - images are all too easily orphaned and owners unknown. Social networking sites strip embedded metadata at will, denying any chance of machine readable content to help find owners. Given that they take rights to use and sub-license your images on posting, I can only guess they don't care.....but I digress.
For the US, there is an agreed format for displaying copyright - I can't remember it off the top of my head but has your name, the © symbol and date.
US copyright registration - this requires creators and authors to submit copies of their work, but this system is optional for assertion of copyright. However, if you have registered images with the Copyright Office and they are infringed then a) The damages can be 5 or 6 figure sums per image and b) given these numbers a lawyer would be interested in talking to you. Anecdotally, I understand that lack of registration means you can only recover damages for what you would have charged* for image use and legal eagles aren't interested in zero rated work - generally speaking of course.
Registering images with the Copyright Office means every image you want registered needs to be submitted. What I do (US residents please help if this is way off advice) is register a batch of images every three months, and the files can be zipped before uploading. You have three months from date of creation to register images, and there are rules for whether the image is published or not, so its worth checking (note to self, re-read rules) beforehand. Registration can be done online and costs $35 per batch. For $140 a year, I get an insurance policy that I hope to never claim on.........but these days who knows?
*Infringement is good reason to have an audit trail of high value image licensing fees to demonstrate similar use. Infringers, if they know their stuff, will claim to see evidence of like-for-like. Sign up for microstock or sell your work at $10 per image and a 'good' infringer will have a chuckle to themselves and pay what was due. Have examples of high value sales and......well....the infringer see things from your perspective with respect to value.