I see.
You retain the rights to the content that you submit to the service, as is written in our terms of service. You do give us permission, so you give us a licence, to distribute it on the service.
This is an issue that Twitter is actually litigating in a court in New York at the moment. A court found that a user did not have standing to contest the provision of their personal information, because the user didn't have any right to or interest in the contents of their Twitter account. So Twitter said, “No, as a matter of fact, under our terms of service, users do retain the right to the content—so their content, including their photos—that they submit to the service”.