Just switched to Firefox, and the problem's gone!
But, why? It doesn't seem to happen to other pictures that I look at in LuLa on the usual system (Internet Explorer), only to my own uploads. Having said which, as most of others' images don't have captions, I don't really have any way of knowing for sure. It's copy where it can be seen as either crisp or definitely not.
Rob C
Tried Google Chrome, and it's okay there, too. Now, with Chrome, as I write, everything's underlined in red! Oh shit. I need a holiday.