When you have to make an image for Kindle you must adhere to the minimum DPI requirements. For Kindle the minimum DPI (or Dots per inch) is set at 300. Bear in mind that it is only a minimum. For many images that will be a sufficient resolution. If, however, we are talking about a cover, or another important image, you may want to go to a higher DPI--450 or 500 DPI would be a better choice, even though it will increase the file size.