I'm not sure if it's my high DPI display or the particular ebooks I have gotten are badly formatted, but the sizes of images are extremely out of proportion with the text zoom or the size of the viewer. Most CSS image instructions set image sizes in absolute terms or relative to the size of the container. transform: scale will end up with images overlapping the text.
To scale all the images in an ebook up without ruining the formatting you can add
To the stylesheet. The zoom level will depend on your DPI.