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Distributors may specify a minimum PPI (pixels per inch) for images. For example, the Amazon Kindle Publishing Guidelines say that your images should have a PPI of at least 300. For full-page image at 4 inches x 6 inches, 300 PPI translates to an image resolution of 1200 px wide by 1800 px high.
Of course, since your book will be displayed at different sizes on different devices, you can't be sure of achieving this. For example, you may comfortably achieve a high density of pixels on a phone, but then someone could display your book on a 50" monitor, and your PPI will drop considerably as the pictures have to be stretched.
So, this is just a guideline and (at the time of writing, September 2018), Amazon doesn't tend to enforce the rule.
Jutoh doesn't explicitly support any kind of PPI concept in images. In isolation, PPI doesn't really have any meaning because it's relative to whatever display is being used and the proportion of the display that the image takes up. So it doesn't make any sense to specify the PPI when saving images, and Jutoh doesn't display an image's PPI even if it's specified within the image file. Instead, choose an image resolution that is reasonable and gives enough detail in the book without unduly increasing the book size.
If you want, you can do a calculation to ensure you're conforming to the guidelines of 300 PPI: you'll have to take into account the size the image appears on the page, the size you impose via the Size tab of the image properties, and a typical display size for which you want there to be lots of image detail.
So, say you have set your image to take up 50% of the display width. You want the image to look good on a display that's, say, 6 inches wide. So the original image should have a width of 0.5 * 6 in * 300 PPI = 900 pixels.
If you're creating a print book as well as an ebook, you may want to insert higher-resolution images. Then, for the ebook configuration(s), you can limit the size of the images by specifying Maximum image width or Maximum image height, in either pixels or as a percentage of the original image depending on the value of Maximum image width units. This will resize (resample) the actual images to create images with less detail and taking up less space in the ebook file.
For more on file size optimizations, please see How can I reduce the file size of my book?
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