That is interesting, thanks Quentin. SO if an ALT tag was empty NVDA wouldn't just speak it, it would not even appear as a graphical object?
You are correct, the site does need you to log in. I've been talking to the IT team behind the website and I'm happy to say they are currently doing a re-design to improve accessibility for screen readers.
The senior designer tells me the images do use the IMG tag but she also tells me it is a slideshow, so I'm just asking her how that might be being done. NVDA is not finding any graphics (except the main website logo) and there are no embedded objects such as YouTube videos.
I'm really happy to see a website taking screen reader accessibility seriously :)