[Plugin: NextGEN gallery] NextGEN can't see images

None of the images in my NextGEN galleries are showing up, either on the live site (eg http://julian-richards.co.uk/early-starings/nggallery/image/turl-street-oxford-october-1987/) or in the back end. Under Manage Galleries the thumbnails are all shown as broken. The plain URL http://julian-richards.co.uk/wp-content/blogs.dir/1/files/early-starings/turl_street.jpg also returns a 404.

I can see the images via the File Manager for my web space -they’re there and where they should be – but for some reason they’re invisible any other way. What have I done?