BookWright will display a warning on an image if it has an image resolution lower than 250 dpi. That means your image was placed onto the page at too large of a size for its resolution. You'll need to make the image smaller on the page for the warning to go away.
If you ignore the warning and don't change anything the printed image will look blocky or pixelated...even if it looks ok on screen.
Your images might be too small if you have downloaded them from the web, 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 to make them bigger and maintain the quality. So your best bet is to reduce the size of the photo on the page until the warning goes away (or let BookWright autofix things...but be sure to review everything afterwards). You may also want try the 200% test on any images you're unsure of.