When you use an image with a Creative Commons license, you have to attribute it in a particular way. Attributing just means saying what the work is, who made it, where it’s from etc.
The acronym TASL can help us remember what to include.
When you use work with a special license called Creative Commons Zero, you don’t have to attribute the work. This is because the creator has released it to the public domain.
So, if you go to a site with CC0 images like Pixabay, Pexels, or Unsplash, you can download the photo and use it however you like. You don’t have to say where it’s from, however, it’s still good online etiquette to do that if you can.