BookWright will display a warning on an image if it has an image resolution lower than 250 dpi. That means that your image is too small for the amount of space that it’s in. This stretching out to fit into the space can result in a printed image that appears blocky or pixelated, even if it looks ok on screen.
Your images might be too small if you have downloaded or copied them from the web, if they come from an older model phone or camera, or if you have the settings on your phone or camera set to save images as a smaller size.
If your images are too small, there’s no way to edit them and make them bigger, so your best bet is to reduce the size of the photo container that the image is in. You may want try the 200% test on any images you're unsure of.