When you try to upload an image it says
User contributions licensed under cc by-sa 3.0 with attribution required (content policy)
My proposal would be to change it to something more along the lines
Reminder: By publishing an image you will be publishing it under a cc by-sa 3.0 license. If you are not the copyright holder of the image, please ensure you have the legal right to publish the image under a cc by-sa 3.0 license.
Yes, it's long, yes it's boring and yes, someone will be able to think of something better, but can we please include some type of reminder that you (or even moderators) can't just publish images you don't own and which haven't been published under a compatible license (even if you give credit).
Just to be clear, I DO NOT expect a small change in text to affect the large majority of users. I do expect however that a very small percentage of users will at some point notice (especially active users) and that they can do their part helping others not break the law. It's more a case of "raising awareness" than "solving the issue".
(Most likely I got at least a couple of those wrong (collected them in just a couple of minutes) and although I reverse googled some of them (to check their license) it is always hard to figure out who owns what... which is exactly why we need to be more careful when we post stuff.)