No need now. I have found the solution.
1) Google Earth appears not to recognise orientation of photos. So if you export a KML file from Avenza maps to Google Earth it turns all portrait images counterclockwise 90 degrees. Horizontal images are fine.
2) I downloaded Windows software called JPG Spinner. Just Brilliant. It rotated all my images in batch mode. I don’t understand how it did it but Google Earth now displays the orientation of my images correctly. Really simple to use too.
3) But after you use JPG Spinner for image manipulation, you must change the Google Earth file (KML) into a KMZ.
4) Then open in Google Earth is everything is correct.
Hope this helps somebody. It took me a few hours of research, trial and error before stumbling up this methodology.