fixed-layout ePub export creating large files--too large for Apple to accept at iTunes Connect

[RockyRakoon]

I've exported a large photo book from InDesign in a fixed-layout ePub for submittal to Apple through iTunesConnect, but it rejects the file because of six images that exceed their 4MB limit.  I've taken courses by Anne-Marie Concepcion and she used ecancrusher to look into and modify epub files, but that apparently was a 32-bit program and I'm using the 64-bit MacOS. Using Calibre I can look into the file and see these six images but they appear to just be large, empty frames, but I don't know how to modify these.

Why would InDesign create such a large blank file?  Is there a means to modify individual png files within the ePub file to allow me to reduce their size?  Is there a 64-bit Mac program that will allow me to unpack, modify, and re-pack an epub file?

Any help would be greatly appreciated.

[Derek Cross · Accepted solution]

BBEdit is good for opening, editing and saving ePubs (it's Mac only). You'd need use (say) Photoshop for optimising your images. TinyPNG/JPG is good for compressing images: https://tinypng.com

https://www.barebones.com/products/bbedit/

[RockyRakoon]

Thank you, Derek.  I've downloaded BBedit and looked into the epub file, and can see the various components there.  But when I select one of the large image files, I can only see gibberish in the BBedit screen, and there does not appear to have a way for me to select one of the images to then open in Photoshop.

I think I need a way to "unzip" the epub file, reduce the size of the five large images, and then re-zip into an epub file again...