title_body: How to improve image heavy mobi format book?
upvoted_answer: Reduce the size of the images using a tool such as free IrfanView or an alternative. If you simply drop an image into a word processor such as LibreOffice Writer or MS Word, and then drag a corner to shrink the image to fit the page, the number of bytes used by the image stays the same. Pre-shrink the image to the approximate size needed in the page layout.
  If you have many images, you can batch process some to a set size with image editors.
  BTW, MOBI files are often twice the size of EPUB, so consider other formats for non-Kindle devices, if file size is an issue.