The Commons on Flickr hosts a number of files from various institutions that state that they have "No known copyright restrictions".

What this means for Wikimedia Commons is unclear, and be aware that some institutions (such as the Smithsonian Institution's Terms of Use) flatly contradict “No known copyright restrictions”, asserting copyright and “No commercial use”. One must thus be cautious about uploading such images to Wikimedia Commons.

See: {{Flickr-no known copyright restrictions}} and mail on commons-l, with discussion by George Oates, Flickr employee heading The Commons, expressing some reservations (The Commons on Flickr announcement).

A great way of finding images on Flickr that can be uploaded to Commons is by using their search tools. You can start on a simple search (limited to freely licensed images), then enter a query.

Or, on the Advanced Search screen, select the "Any license" option and then choose "Commercial use & mods allowed" from the menu so that the setting reads "Commercial use & mods allowed" instead of the "Any license" default. You can also search by specific licenses, such as by-2.0 or by-sa-2.0.

Magnus' FlickrFree tool shows the last 500 free-license images uploaded to Flickr and lets you easily import them to Commons.

Also the {{Flickr free}} template lets you search for images under either license.

If a file on Flickr is not licensed under CC-BY (Attribution) or CC-BY-SA (Attribution and Share Alike) or CC0 (Public Domain Dedication) or marked with the Public Domain Mark, it is usually not allowed on Commons. However, Flickr users can change the license of their work. Read Flickr's guide about how to change licensing on a single photo or multiple photos. Commons users can also ask Flickr users to release their images under a Commons-compatible license. Such requests are best made by sending a mail to the Flickr users from within the Flickr website. You need a Flickr account to do this. For more help, see Commons:Flickr files/Appeal for license change.

It is possible that the license of a CC-BY(-SA) or CC0 or PDM image may be changed on Flickr after it has been uploaded to Commons and verified. Flickr users are allowed to change the licenses of their works. It is important to note, though, that once a work has been made available under a given license, the copyright holder cannot legally change or revoke the license on copies of that work. Thus, an image originally licensed as CC-BY and uploaded to Commons under that license (and verified), but later changed on Flickr to All Rights Reserved, may remain on Commons under the CC-BY license. Such a situation is undesirable, of course, and politely asking the Flickr user to reconsider may be a good idea. In the meantime, you may tag the file with {{Flickr-change-of-license}} to indicate the license disparity.

If a Flickr user changes their work's license to a freer version (e.g. CC-BY-SA to CC-BY), the copy on Commons may be updated to use this new license. Simply remove the existing Flickr review template and request a new review with {{Flickrreview}}. As above, however, if the Flickr user changes the license to a less-free version (e.g. CC-BY to CC-BY-SA), Commons is not required to match this as the freer license is not revocable.

Shortcut: Use Magnus' upload tool (described below)