Website MyFreeCopyright.com provides a verifiable historical public record asserting your copyright to works you create.

MyFreeCopyright creates an electronic fingerprint of your image, audio, or written creation and emails you that signature, thereby providing both a unique key to identify the work and a time-stamp to verify the date of creation should you ever need to defend your work.

Submitting your work to deviantArt.com provides a similar proof of authorship (in that the date of publication is recorded by a third party), but MyFreeCopyright.com has the benefit of allowing anyone to submit a copy of the image without knowing who you are, and the site will match the signature with the copyright owner. (Caveat: Only unedited works will actually match. You will need the original file to defend your creations.)

The first 100 works you submit are free; additional images may be registered as part of their annual subscription service, as are additional features related to archiving copies of your work. (features/pricing) Subscription prices are about the same level as a deviantArt subscription account, and certainly worth considering as an alternative to more expensive watermarking services like Digimarc's MyPictureMarc.