I have recently sent my last picture using Twitpic, the photo sharing service on Twitter, because I was made aware of an important change in their Terms of Service:
You retain all ownership rights to Content uploaded to Twitpic. However, by submitting Content to Twitpic, you hereby grant Twitpic a worldwide, non-exclusive, royalty-free, sublicenseable and transferable license to use, reproduce, distribute, prepare derivative works of, display, and perform the Content in connection with the Service and Twitpic's (and its successors' and affiliates') business, including without limitation for promoting and redistributing part or all of the Service (and derivative works thereof) in any media formats and through any media channels.
I suggest using MobyPicture.com, or even better PhotoShelter, both sites offers terms of service that are much friendlier to photographers.
