[Starson17]

This code will rotate images so that the long dimension is vertical. It's been updated and revised from code found here after python magick wand was replaced by calibre.utils.magick and calibre.utils.magick.draw:

The code would typically be used for image-based recipes (comics) read on devices that don't have a g-sensor based autorotation capability or where you prefer to lock your reader software into portrait orientation. This code makes the long dimension of the image appear in the long dimension of the screen on a handheld device that can be rotated for better viewing. Don't use it for recipes being read on a non-rotatable PC monitor (unless you want to view images with your head tilted 90 degrees).

You may also want to set extra_css to include something like:
img {max-height:95%; min-height:95%;}
This leaves some room above and below the image for the links typically displayed in recipes.

Spoiler:

#Add these imports
from calibre.utils.magick import Image, PixelWand

    def postprocess_html(self, soup, first):
        #process all the images. assumes that the new html has the correct path
        for tag in soup.findAll(lambda tag: tag.name.lower()=='img' and tag.has_key('src')):
            iurl = tag['src']
            img = Image()
            img.open(iurl)
            width, height = img.size
            print 'img is: ', iurl, 'width is: ', width, 'height is: ', height 
            if img < 0:
                raise RuntimeError('Out of memory')
            pw = PixelWand()
            if( width > height ) :
                print 'Rotate image'
                img.rotate(pw, -90)
                img.save(iurl)
        return soup