There's a thing called Fair Use. https://en.wikipedia.org/wiki/Fair_use
That's some technical shit but it's good to know.
I've seen people write music reviews and include a video. But if someone just posts an Eminem video without saying anything, they're straight up attempting to monetize shit they don't own.
Some images found online can be used provided you give credit. Always give credit if it's not yours. Browse around and look at some posts. You'll find some eventually that show 'source' under images. Just make it clear you're not intending to claim ownership.