WEEK 28: CREATIVE COMMONS
Therefore Wikipedia allows uploading of images and other media so long as they are published under one of two CC licenses:
Attribution (CC-BY-SA) — This means you are publishing something for others to use for any reason (including commercially) with the sole proviso that they credit you for the original.
Attribution ShareAlike (CC-BY-SA) — Same as above, with the added stipulation that anything created using your work is likewise released under this very license as well.
Creative Commons is effectively halfway between "all rights reserved" or "no rights reserved" (public domain) but it also allows rightsholders to grant permissions more specifically. Be forewarned: it's kind of a tricky subject! But you can always ask for help.