De-lurking here.

I have a ZP installation that's integrated into a web site. I have modified the image.php and album.php pages as a result of this, and all is well, however....

I had written image descriptions into the ITPC field when I edited the images. I looked: the descriptions are there in the MySQL table in the "desc" field, so that looks right. The descriptions also show up in the ZP admin panel. BUT, the descriptions don't appear on the image pages.

I had a look at the code, and it appears that these lines SHOULD make the description appear:

Have I inadvertently taken out some other lines that control the display of the description?

Yes, you have, those lines are plain html that does nothing...:-) You need to have something like printImageDesc() in our theme page. Please see one of the standard themes how it is done there and/or the theming tutorial.