It’s not hard to find images to use online – just use an image search. This works well, but it’s worth noting that doing so isn’t necessarily legal. Most of the images you can find this way are owned by their original creators. This likely doesn’t matter if you’re only using something for private use, or even a school assignment, but if you intend to publish a work you need to make sure all rights are cleared.
Is there a potential downside? Yes. Just because a search engine sees something as Creative Commons doesn’t necessarily mean that it is. Here at MakeUseOf, for example, we’ve had at least one photographer threaten to sue us over an image he didn’t realize he himself licensed as Creative Content. He backed down when we pointed this out, and it’s one example resulting from thousands of blog posts spread over a decade, but know that this isn’t without risk.