A lot of people read on e-ink ereaders, so they would only be getting black and white images. They're probably not going to want to pay extra for that. As a suspense reader I definitely wouldn't pay extra for pictures in suspense novels, unless it was for something like the Illustrated DaVinci Code where the print version was extra large and printed in color on thick glossy paper.
Something to keep in mind, adding pictures to your ebook will add to your file size, which will in turn add to your download costs.
Also, inserting even black and white illustrations to an eBook is a tricky premise, and Zon help will give you the standard response, which, although helpful, doesn't always fix the fact that your illustrations will show up in your eBook as big camera icons, i.e. -- no illustration.
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