Today I was working with a rather large EPUB exported from InDesign CS5.5. The final compressed size of the EPUB was 18MB; most of this size was comprised of 206 PNG images. In attempt to reduce file size, I ran the images through a couple batches in Photoshop with little result. I then searched the net and found a free utility called PNGenie claiming lossless size reduction of PNGs, and allowing drag-drop of multiple images simultaneously.
So I downloaded it and gave it a try. On first run it must download an additional component and install it; then you simply drag and drop your images. It took about 1 hour to process all 206 images. The combined file size of the images went from 19.2MB to 10.1MB. I opened one of the before/after images and compared the pixels, and they looked identical to me as far as I could tell, so there was no visible reduction in quality.
This nifty free utility just cut my EPUB file size in half. So as far as I’m concerned, it’s a winner!
If you make EPUBs from InDesign, you may benefit from it as well. It seems that the image converter InDesign uses to create its PNGs during EPUB export adds unnecessary bulk to the files, and if you have a lot of them, you can really reduce file size by running them through this utility. Just unzip your EPUB, run your images through PNGenie, then zip it back up using ePubZip. Voila! You have a much smaller EPUB, with no loss of quality.
You can get PNGenie here: