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Sorry, I got the impression you were talking about comics.
The only automated way I can think of is using calibre to do a conversion. The Output Profile can be used to set the maximum size of the images. But, it only changes the size of images larger than this. I don't see an option to change the images other than this.
If you just want to change the images to grey scale or otherwise fiddle with the image quality, you can unpack the book and use image manipulation tools on the images and then repack the images. Calibre has a method to make the unpacking/repacking easy. There has been some discussion on doing this in either the calibre forum or the Workshop forum. Searching them should find them.
If you want to resize the images, it is harder. From my experience, the images are embedded in different ways, and each book, or even each image, needs to be handled individually.
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