If you have an artist illustrating your book, and you want to make it into a cartoon, ask them to also save the characters and background art separately, as transparent png's, along with the jpeg's that you'll use to publish the book. It's not hard for them to do, the only difference pretty much is that you save the art as a png versus saving it as a jpeg in Photoshop or whatever image editor you're using. Photopea is a free image editor, so you don't even need Photoshop.
Here's an example of what a png looks like:
It's really easy to make a png, you just copy and paste. I copied and pasted Torrie the Tiny Sea Turtle from this illustration on the left and saved her as a transparent png on the right (as opposed to a jpeg, which is what people are mostly familiar with). You can see the background of her separate image appears as little white and grey squares, to indicate she's transparent. This lets me paste her onto different backgrounds to make her move.
That's how easy animation is, you just copy and paste whatever you want to move, her eyes, flippers, tail, etc., onto a separate png layer!
P.S. My husband, an illustrator of many books, just told me a png is the clearest image you can get for things like headers on fb, Youtube, etc., so if you're looking to replace your header, that's also a good thing to use, versus a jpeg.