Last few years I uploaded a couple illustrations to my zenPHOTO (1.4.14) site that are 9201x6800, but now (if ever) they're not displaying. Can anyone state what adjustments one needs to make (maybe to php.ini?) Though, they're black & white (or greyscale,) each under 8MB.

I'm not surprised. Those are huge images.
Please read this article carefully:
http://www.zenphoto.org/news/problems-with-albums-and-images/

nah, these are very small (9x12 inch) images. Even real medium-sized art (might take up a house room wall, not, for example, one large like the Sistine Chapel) may not be scanned easily with contemporary technology (though almost.) So, it's only large for average/old PCs, not in general (I mean, I've edited much larger stuff on my PC.) I've already read that article carefully, but it needs updating. My server is 10 years old and has way more than enough dedicated RAM (4GB) to display one of those, but the article isn't up-to-date enough.

note, as stated, each image is under eight megabytes (since black & white,) closer to lower-range in that article. If they can be stored permanently in that small size (fits on a few old diskettes) seems there's something going wrong with how things are done in the temporary (display/video/graphics card) storage

Please closely review the link my colleague posted above. It is not the file size that matters but the resolution of the file itself. A huge JPEGcan be compressed to be very small. This is generally a server memory issue as stated (and this part is by no means outdated). Another source of the problem could be corrupt meta data.

Without any proper error message we are just guessing. So review your server logs please.

Resolution list is outdated: since decades ago, I regularly edit 800DPI 11x17 inches² images, easily, for years (relatively small in overall reality of art.) If I get back to editing more than just comic-book-page-size (like more artists) it'd be 800DPI largest watercolour-paper-size pieces (several-feet-long-sides) or fitting largest flatbed scanners (might need a DVD or BD to hold results)... someday probably something medium-sized (need a wall-mounted scanner.) Likely then it is a server issue... will have to research logs (more) and/or metadata...

The resolution list may be outdated as image may be larger nowadays. But the core problem with servers not capable is still the same.