First, understand what is the recommended image resolution for placed images. The printer would recommend 300dpi, higher or lower depending on the printing machines and techniques. Supposing the recommended resolution for your printer is 300dpi:
- Select the placed image in InDesign and go to the Links panel and note the "effective dpi" shown in the panel. This should fall within the range what the Printer recommended.
- If it is not matching with the recommended resolution, use image editing program to change the resolution, or use alternate images of right resolution.
- If the image resolution has been tampered due to scaling in InDesign, avoid such scaling which takes the images to unprintable resolutions.
Note that if the images are not within the printer recommended resolutions, the images would still print from InDesign, but the output quality would be affected.