Depending on how many images you have in your book, and how large the final file is, it may be timing out the import process before the images can fully make it into the program. Whenever working with images, our best recommendation is to have them all saved to a separate folder and removed from your .docx file.
Once the manuscript itself is uploaded into Atticus, you can insert the images exactly where you want them. We have a great tutorial to help you insert and adjust images in a variety of ways here: https://www.atticus.io/interior-book-images/