I frequently have clients that provide images for their books, and many times they are not high enough resolution for printing. “But it looks fine when I print it at home,” they say, “why won’t it look good in my paperback?” The reason is that printing presses do not use inkjet printers.

If you look at a picture in a book with a magnifying glass, you will see that it is created out of little dots, and printers require there to be 300 of those little dots per square inch of picture, or 300 DPI. A monitor or device screen typically displays pictures at 72 PPI, Pixels Per Inch, and sometimes as high as 150 PPI.

DPI, Dots Per Inch, and PPI, Pixels Per Inch, are essentially the same thing. A picture that is downloaded from the internet is typically 72 PPI. This works well for internet use because it keeps the file size small. It will also work well for your ebook, but it won’t be useable for your printed book. If your images are less than 300 DPI, the software will have to make up the rest of the dots, so it will look blurry, and your printing company will not accept the file.

The other important aspect of resolution is that the pictures must be 300 DPI at the size they will be placed on the page. If I have a photo that is one square inch, it has 300 pixels. Since that is a little bitty picture on an 6x9 inch book, I would want to enlarge it. If I make it into a two inch by two inch picture on the page, it will still only have 300 pixels, which will make it look blurry. If I have a larger picture at 300 PPI, I can reduce it any amount, because there will be plenty of pixels to render the picture. Likewise, if you supply me with a very large picture that is 72 PPI, I can raise the resolution by reducing the overall size of the picture but only to the point where it is no less than 300 DPI. To be flexible in design, it is best to supply your designer with the largest picture at the highest resolution possible.