We support PNG, JPEG and GIF formats for images. (To be clear, we don't support SVG images.) If you're writing in Microsoft Word, you can just paste images into your Word file, and they'll show up in your book when you create a new preview or publish a new version. If you're writing in plain text, first, put your image into the images folder in your book's manuscript folder. Then you can get the image into your book by writing a line in your text naming the image, like this:
That's it! The next time you create a new preview or publish a new version of your book, the image will show up. (See this web page for details on images in Markdown.).