Thanks for the reply Fred.

Trouble is that this is an existing system so I don't want to change
it too much to avoid alienating the users who built it in the first
place. The other problem is that we don't have an enduring solution
for storage of the database so it'll have to move in the near future
so separate images would make this more difficult.

I think I can cope with the issue of bloat by making sure that the
images aren't 12 megapixels but something more appropriate for a 4
inch preview.