Many services that host creative content, such as YouTube, flickr, Vimeo, Wikipedia to name just a few allow users to select a creative commons license for their works.
For example when uploading a video to YouTube it will automatically apply the Standard YouTube licence. If you are logged into your account, you can change this by clicking on the ‘Edit Video’ button underneath your video and then clicking on the ‘Advanced’ tab. Click on the dropdown box ‘License and rights ownership’ and select the ‘Creative Commons – Attribution’ option. This will then mark your content as CC BY.