Print books traditionally request a minimum image resolution for interior images to be at 300DPI. DPI stands for "dots per inch," and you can usually see the DPI (or resolution) quality of an image by looking at the image's properties in your file browser (in Mac, "Finder;" in PC, File Explorer).

Some (Print on Demand) companies will allow you to print a book with images as low as 200DPI.  Createspace (update: defunct as of late 2018) will, for example—but IngramSpark, as of the last time we checked, will not.  Nor will KDP Print.  If you try to submit a book with a 150 or 96DPI image in it, you'll receive an error notification, warning you not to proceed with printing the book. At KDP, they don't simply warn you—they won't allow you to proceed.

How Do I Solve an Image Quality Issue?

Ideally, the solution is that you just used the wrong image, or that you put your image in Word, and Word compressed it, losing resolution in the process.  If Word mashed your image, then read our article, here in the UBERQ, about how to stop Word from compressing your images.  If we've already made your eBook, or print book, then just send us the full-quality images, and voilà, it's handled.  

But, What If All My Images are Low Quality?

Well, that's a different situation.  There are different solutions, not all being equal, unfortunately.  You can:

1.Revamp your file, and find better-quality images to replace the ones you have;

2.Omit the poor-quality images altogether;

3.Reprocess the images at a "conversion" or "up-rezzing" website like Convert Town.  Now, all this does is increase the perceived resolution of the image, so that the automatic checking software at a place like KDP Print won't choke on it—but failing better options, this is what you can do.

4.Change up your file, and use different, better-quality images.

These are really the only options.  We are experts at image reprocessing and preprocessing, but once an image has been made, it only has so many pixels—and we can't make pixels out of thin air, sorry.