On a whim, I looked at the first picture in an Album which
had this problem in iPhoto. I had, I guess, edited it, because
it gave me the option of reverting to the original.
I did that, and used a different browser to ensure I had
a fresh cache and that picture then appeared under
wipha.

I'd venture to suggest then that if a picture is edited
then there is an issue, but a) it doesn't seem to
be consistent in that I'm sure some pictures I've
edited are visible, and b) there don't seem to be any
aliases or symlinks in that directory which would suggest
that perhaps something might also be screwy in the
xml file if it shows the associations.
So, I moved the Album xml file to the desktop.
It was auto regenerated (and different) but that didn't
stop the problem.
I watched the error log under httpd and I believe I have
the culprit.
I used to run myPhoto before wipha. Under myPhoto
my iPhoto library lived in a subdirectory of Sites
and there was an alias from Pictures/iPhoto Library to
that folder.

Now, when wipha wants to access some pics
(either thumbnails or full size) the file can't be
statted because it contains a mangled path, such as

stat /Users/isaac/Pictures/iPhoto Library//Users/isaac/Sites/myPhoto/iPhoto Library/2006/02/23/IMG_3486.J: No such file or directory

I'm guessing then that some other file in the iPhoto Library
needs to be rebuilt, although I'd be hoping that I retain all my albums
etc of course.

Does this sound plausible?