: Re: 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

@Radia

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I had a similar situation: a 134 MB .mobi that I wanted to shrink.

Here's how I reduced the file size to 26 MB. The key to this approach is that an .epub is actually just a zipped web page with image files in it.

Converted the .mobi to .epub with Calibre.
Renamed the .epub to .zip.
Extracted the .zip. Note the images subfolder. Mine had more than 1000 files in it.
Used Photoshop (although any decent image editing app could do it) to do a batch job against the contents of that folder. Saved each file as the original file name, but as a low quality .jpg. (In my case, the original filenames were .jpeg, but Photoshop saved the new files as .jpg, so I had to delete the original .jpegs and rename the new ones from .jpg to .jpeg after the batch was completed. There's probably a way to avoid that step.)
Re-zipped it.
Renamed to .epub. I wanted an .epub as the end result, so I was done. But you could of course now use Calibre to convert to whatever format you want.

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