You’ll see the little CC badges on images all over the web. Most people click past them without reading what they mean. That’s how mistakes happen.
The basic one is CC BY. You can use the image, change it and share it, even commercially, as long as you credit the creator. Add SA, for share-alike, and anything you make from it has to carry the same license. Add NC, for non-commercial, and you can’t use it to make money. Add ND, for no derivatives, and you can share it but you can’t edit or crop it.
The odd one out
CC0 isn’t a license the same way. The creator gives up their rights as far as the law allows, so you can use the work with no credit and no conditions. Some of the biggest museum collections now use it.
How to credit properly
A good credit has four things: the title of the work, the author, where it came from, and the license, with a link to the license text. Put it right under the image or in a clear credits section.
Two warnings. “Non-commercial” is fuzzier than it sounds, and an ad-supported site may not qualify. And a license only covers what the licensor actually owns, so check that the person who uploaded it really took the photo.