Looking for cartoons for your book? Check out https://www.cartoonstock.com/
I personally use Noun Project and Unsplash for free images and historically have used Crestock & DepositPhotos for paid. As more companies have gotten into this business, the prices have dropped.
When it comes to designing your PubWriter website, I find it best to incorporate a client's branding into any vector artwork I find at Noun Project. The vector images allow us scale and color images to exact color pallets of the website we build.
Note: When you hire us to create a book cover for you, we'll provide you with a list of sites you can select an image from (and we won't charge you for the stock image if you use one of the sites we provide).
Here are some sites that offer free images, but a word of caution: be sure you have the rights to use it in a commercially sellable product (ie. a book).
If I am just looking for random safe images to use, I usually start my search here. You can't just grab images you find off Google images, you don't have the rights to use those images and there are companies who make a living by fining people who use them without permission.
There might be some duplicates from what I listed above. Also, sites change their terms all the time, so be sure to double check before you start using an image you find on a website.