if you want to email me a copy of the theme I can see if I can find anything. No promises and I wouldn’t hope too much 🙂 But willing to take a look.
Andy, you can download the theme here: http://presscustomizr.com/customizr/
I am using the Pro version, which is linked to a URL (mine), therefore you wouldn’t be able to use it.
But the free version should be very very similar, I hope…
In the meantime, the boys who designed the theme asked me a copy of Simple Press, to make some tests as well 
Done… and afraid to say I have no problem with popups generally and images in particular. I guess this could have something to do with the pro version but I am finding no problem. Sorry!
Problem solved, Andy!
It was a plugin called “Ultimate social media plus”.
I just removed it and now everything is working fine.
Thank you for your help!