There are a lot of clipart images available on the web if you're willing to pay a fee, but there are also some free options!
If someone uploads an image to one of these websites and you want to double check to make sure no one else claims to own that image, you can go to Google Images and click the camera icon to do a reverse image search!
Clipart/Stock/Vector Images
Remember: There are some restrictions when publishing these images, for example when using photos of people clearly featured, you can't do so in a way that implies they endorse something they don't. For more information about restrictions check out this blog post.
Pixabay
All images and videos on Pixabay are released free of copyrights under Creative Commons CC0. You may download, modify, distribute, and use them royalty free for anything you like, even in commercial applications. Attribution is not required.
Pexels
All the banner images on this Lesley guide were pulled from Pexels! All photos on Pexels are licensed under the Creative Commons Zero (CC0) license. This means the pictures are completely free to be used for any legal purpose.
Unsplash
By using the images you agree to their terms of service: Photos cannot be sold without significant modification. Compiling photos from Unsplash to replicate a similar or competing service.
Public Domain Images
You can also use Public Domain images for free: