title_body: How can I reduce the resolution of images in an epub or mobi file? Some of the last O'Reilly ebooks I got have become really large (e.g. 60MB for the mobi version and 5.5MB for the epub version) - and these are computer science text books (not art books). The customer support claims that this is o.k. and they want to provide high quality images. 
  
  Is there software that can easily reduce the size of images in an epub or mobi by e.g. downsampling them? Is there a calibre plugin for this?
upvoted_answer: Calibre already offers this functionality without any plugin. It's part of the ebook editor that installs with Calibre. 
  
  The editor will only edit files in the epub or azw3 format though. You can convert the mobi file to azw3 and then edit it. Converting it from mobi to azw3 may automatically downsize the file, but I'm not sure. 
  
  Once you have the file opened in Calibre's ebook editor, click on Tools, then click "Compress images losslessly". 
  
  Calibre &gt; click the ebook &gt; Edit book &gt; Tools &gt; "Compress images losslessly"
  
  This will bring up a dialog where you can accept the default compression or choose your own. The downside to this is that it only compresses images that are in the jpg format.