Thanks for the quick reply, Ron.
I was using Safari + Voiceover on iPhone. When it's reading everything on the page (swipe up two fingers), it still reads the picture file name and 'link' so you are right, it's perceiving it as a link and therefore reading it as such.
I did as you advised and added the Lightbox optional property, then unchecked it, and it is now behaving as expected.
I've also noted that if the Lightbox optional property is added to the whole project, and unchecked, that we can disable it for all the images so that is very good to know.
Many thanks!