[Quoth]

Also another thought, unless you have fixed layout, splitting an image across tw0 pages even when a reading app or ereader can display two pages at once doesn't work because if the font is increased or decreased when the two pages & images are side by side the reflow will mean the pair of images are split between opposite sides of a virtual leaf.

Fixed layout epub is horrible. If you need fixed layout then make a PDF. But a reflowable epub is better and simply don't split an image into two halves, or else combine them if the paper has them as separate images (but likely they started as a single image. If only some landscape images need to fill the device screen they can be edited in advance to 90° rotation and fill screen in portrait mode via width: auto and height: 100%, then the text can be read in portrait mode.

Many but not all apps and ereader allow landscape display, but if the original intended for paper is mostly 2 page spreads, then assume person reads in landscape and combine each pair of source images as single landscape images with width: 95% (or 100%) and height: auto. If the app or ereader doesn't do landscape, or the user doesn't know how to do that, then the images are smaller. You can remind readers at the start to select landscape display and rotate the device. Some apps and ereaders will automatically rotate the content.