I've just come across the same exact problem.
And <img> with position:absolute contained in a <div> is showing ok in ereaders and Android reading apps.
However, iPad decides to not bother with it and show it blank. As soon as the position:absolute is removed the image is shown by the iPad. However, it is not vertically centred as that's why I was using the absolute positioning
Thank you Apple for making our lives more difficult