1. Check the resolution of the images to make sure they are large enough for the document size.
2. Resize the images to match the document size.
3. Make sure images are saved as a standard format like JPEG or PNG.
4. Adjust the DPI (dots per inch/ dots per cm) to the desired resolution; higher DPI will give better quality.
5. Finally, check that the image has been compressed properly with the correct optimization settings.