John, Lucy: You're stars! I have now done so, upgrading to SP3, and it's
fixed the problem!

It took me a few hours because I hit a major snag. I first tried it on my
PC, updating in 2 stages to SP2 then SP3, successfully. But when I tried to
repeat on my wife's system, I was told it wasn't even yet at SR1 level. I
quickly found the 'SR1a update' - but that turned out to be an *online*
version. On Janet's dial-up connection, that would have required hours.
Eventually, after much searching, I found O2ksr1adl.exe at
http://www.microsoft.com/office/orkarchive/2000ddl.htm
and downloaded the  57 MB file on my broadband, and transferred it across on
a flash drive. I was then able to do the 3 updates. At the end of which I'm
delighted to say it seems all those errant files now display images which
have survived maximise, restore, page changes, etc.

We'll do some more exhaustive testing when Janet gets home from school, but
I'm really confident this has done the trick. Thanks again for your
breakthrough help.