In most cases, you can design images based on the print trim size at 300 DPI, as eBook images are automatically resized to fit the available space. If you have a large number of images, particularly multiple images with file sizes exceeding 100KB, you might consider creating a separate ePub version with smaller images. You might also consider image compression before importing images. While Atticus will do some automatic compression upon export, it's limited to avoid reducing image quality. If you compress your images outside of Atticus, you can exercise more control over how much each image is compressed and check the quality before uploading it to your book.
Compressed images can also be easier to work with because they take less time to render and process.
For more information on sizing your images, you can find more details and calculators in this tutorial: https://www.atticus.io/calculate-image-size-for-print-and-ebooks/