When you transfer Subscriber Content to Tumblr through the Services, you give Tumblr a non-exclusive, worldwide, royalty-free, sublicensable, transferable right and license to use, host, store, cache, reproduce, publish, display (publicly or otherwise), perform (publicly or otherwise), distribute, transmit, modify, adapt (including, without limitation, in order to conform it to the requirements of any networks, devices, services, or media through which the Services are available), and create derivative works of (including, without limitation, by Reblogging, as defined below), such Subscriber Content. The rights you grant in this license are for the limited purpose of operating the Services in accordance with their functionality, improving the Services, and allowing Tumblr to develop new Services. The reference in this license to “derivative works” is not intended to give Tumblr itself a right to make substantive editorial changes or derivations, but does enable Tumblr Subscribers to redistribute Subscriber Content from one Tumblr blog to another in a manner that allows Subscribers to, e.g., add their own text or other Content before or after your Subscriber Content (“Reblogging“).
When you upload your creations to Tumblr, you grant us a license to make that content available in the ways you’d expect from using our services (for example, via your blog, RSS, the Tumblr Dashboard, etc.). We never want to do anything with your content that surprises you.
One thing you should consider before posting: When you make something publicly available on the Internet, it becomes practically impossible to take down all copies of it.
Anyhow, good to know.