Scribe theme – Cannot pick image / Select background image



I am very new to Upfront and the Scribe theme. I would like to swap out the background image that comes with the Home Hero top section and swap it in for an image of our choice. I can see how to edit this background color and change the image, but when I select background type – Image and then try to pick the image, the editor freezes in a blurred view and doesn't ever open a dialogue box that lets me choose an image – or do anything else (see screen shot). If I refresh the page I can get back to the editor, but otherwise the whole process seems to get stuck there. I have tried this on Firefox and in Chrome with the same problem. Please could you advise what the problem might be?

Many thanks,