I backup my photos online using Amazon S3 services. You'll need to use a third party program like
http://www.jungledisk.com to access your S3 account, but once you've configured everything it's easy to upload and retrieve your files.
Amazon S3 only charges you for what you use and the prices are very very reasonable.
http://aws.amazon.com/s3/
prices:
http://aws.amazon.com/s3/#pricing