URL is:
http://www.composersofwales.org/
The jpegs in question are buttons on the left, contained within an INCLUDED file (exactly the same as the image across the top which, on the other hand, does show. I haven't touched them, and they're definitely sitting up there on the site because I can go to them in a browser window. It may be something to do with the include file, I suppose, but I can't see anything wrong there - they show up on the local version of the file, just not on the web.