There are a few ways to handle this depending on where the photo comes from. For the most part, you should make every effort to identify a fair use photo and to credit its original owner.
If you got a photo from Kim Kardashian’s Instagram account from people.com, you would credit Kim Kardashian’s Instagram -- NOT people.com.
If you got a photo from a photo library like Pixabay, Photopin or Wikimedia Commons, you may credit the library itself, unless the license requires you to also attribute the author (e.g., Gage Skidmore, Wikimedia Commons). However, in most cases, attributing the library with a link to the photo’s page is sufficient.
If you use official promotional still from TV shows or movies, credit the channel or movie studio they came from. Exceptions include photos exclusively released to magazines like Entertainment Weekly (they will often bear a watermark of the publication).
If a photo is marked “public domain,” no credit is necessary (but you should still denote it as “public domain”). Photos taken by government employees are considered public domain.
Whoah! Under “Content Requirements,” this assignment says I don’t have to source images! What the what?
That means this particular client doesn’t care about copyright fines and you can use whatever photos you want to make the assignment work. They’re taking full responsibility for their photos, it’s not on us!