Problem with pictures

Hi,
Firstly, just to say thanks for such a great media center. I've already earned kudos with my wife for doing something so useful with a Raspberry Pi.
I'm running Raspbmc, it's the new, final build. I installed this yesterday and everything seems to work fine except pictures.
Our photos live on an A300 "popcorn hour" upnp AV server and while music and video streams no problem, trying to display a picture just shows a black screen on the TV.
To get out I have to press menu on the remote.
It can play pictures from other sources, such as Picassa okay but just seem to show a black screen when they are from a local source. I can see file information for the pictures etc. The picture itself wont show.
I have tried disabling overscan except for video playback (made no difference) but my web trawling has otherwise yielded nothing.
Would appreciate any help as it's the only thing wrong with an otherwise brilliant system.
Thanks