Currently, you can't directly add images to your image gallery in Atticus. However, the image gallery automatically accumulates all the images imported into your book.
When you add an image to your book, you will encounter two options. One is labeled "My Image Gallery," and the other is "Upload a New Image." To upload a new image, click on the gray box under the "Upload an Image" option. You can either browse for files on your computer or simply drag and drop a file into this gray box.
Once you've uploaded the file into your book, it will be stored in the image gallery, making it easily accessible for future use. If you need any further information on working with images in Atticus, I would suggest checking out this tutorial: https://www.atticus.io/interior-book-images/