What it means
DPI (dots per inch) describes how finely a printer lays ink; PPI (pixels per inch) describes how much image data exists for each printed inch. The two are used interchangeably, and what a book printer cares about is the second: an image should supply 300 pixels for every inch it will be printed. Below about 200 ppi photographs soften; below 150 they look pixelated. Line art and text as images need more — 600 to 1,200 ppi — because hard edges show every step.
The arithmetic is simple. A photo printed 6 in wide at 300 ppi needs at least 1,800 pixels across; a full-bleed 8.5 × 8.5 in picture-book page needs about 2,600 × 2,600. Resolution is a ratio, not a property of the file: the same 1,800-pixel image is 300 ppi at 6 in and only 150 ppi at 12 in.
Upscaling in software invents pixels; it cannot add detail. Start from the largest original you have.
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