tl;dr: Right now we're not allowed to upload any images we don't own (fair use does not apply on this site, see the link). Let's make this a bit clearer in the image upload form.
When you try to upload an image it says
User contributions licensed under cc by-sa 4.0 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 4.0 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 4.0 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 might (not obligatory!) 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.)
After thinking about this for half a year, alternatively SE could also just openly ask in a dialog box when you try to upload
Do you have the legal right to publish this image under a CC BY-SA 4.0 license? Unless you created the image or the image is in the public domain the answer is typically "no".
It's in your face, but at least everybody will notice this.