The resolution required for a photo in a photo book ultimately depends on the size of the image area. If one of your image files doesn't have a high enough resolution, the photo book designer will automatically provide a warning in the form of a small red circle with an exclamation mark. The affected photo will be marked with this warning in the top right corner, allowing you to remove or replace it. Please note that the system does not detect blurriness; it only analyzes the technical resolution of the image.
In the photo book designer, you also have the option to apply filters and adjust the image to your liking. Simply click on the desired image to open a menu directly below it. By clicking the "Adjust Image" button, you'll access filters and other editing options like brightness, contrast, etc.