When adding images to your book, there are two things to think about — if you have permission to use the image, and if the image will look sharp and clear when printed.
If you didn't create the image or take the photo yourself, you need permission from the copyright owner. It isn't enough to namecheck the image owner in your book, or to assume that an image is free to use if there is no mention of rights alongside it.
A way to avoid needing to seek permission is to look for licensed images. A license tells you what you can do with an image. You don't need to get permission because what the copyright holder will or won't let you do is explained in the license.
You can tell an image is licensed because it will have the conditions for reuse beside it or in the image caption. There are also sites like Unsplash where all images on the site share one license, so you know already what you can do with any image you find.
Your image will look sharp and clear if it has a high enough resolution. Most images you find online look great on-screen but have a resolution that is too low for printing. That's because printing on paper is capable of showing much more detail than your screen can. On screen, images don't need to have a high resolution to look great. Keeping the resolution low also means smaller file sizes, allowing pages to load faster.
Always look for the highest quality image possible. If you're unsure, pick the image with the largest file size. You can also message us and share the image you'd like to use. We can re-size it to the size it will be in your book, and tell you how it will print.