XnViewMP is both a image/photo viewer and file manager, plus it also has most editing functions built-in.
digiKam is popular photo manager.

If a user is not using kmail or any "pim" associated applications in kde, akonadi can be uninstalled.
However, that does not remove the akonadi files in /usr/bin. You have to do that "by hand," to get rid of akonadi. kde will run just fine without it.

A tad off-topic but I wade through my 5k+ photos using Geeqie - light and fast photo manager and views the images on disk without sucking them into a db.

I just accidentally emptied two video files (reduced them to zero length) trying to launch them from nnn.

I'm still trying to figure out how it happened, but at one point the the process, the word "Archive" was in the status line.

EDIT: Figured it out. I'm entering an issue on GitHub.

EDIT 2: It's fixed in 2.9. What happened was that in 2.8, if you pressed "f" ("archive") (which is also "fullscreen" in most video players) and then accepted the default choice for the archive name, which was the name of the file, nnn would truncate the file to zero length. 2.9 doesn't have this problem.

In the "olden" days I used 'xv'. It had similar (though, probably much less sophisticated) capabilities.

I tried digiKam years ago but didn't much like it. Maybe it's gotten better. For some odd reason, digiKam wound up getting loaded onto a server I built a a couple of years ago for email and file serving---it seems it's really trying to be ubiquitous. (I'm unsure what component I requested at installation that dragged it onto the hard disk but I uninstalled it.) I just file photos into directories ("yyyy/mm/dd") using the EXIF date info. I typically have emails that identify event dates when photos were taken so I don't need a special database with tags, etc. Coming home from a sporting event with 700 photos and feeling like I have to tag them along with the cropping I might need/want to do (and then tagging those edited versions) is, IMHO, an unnecessary task I'd just as soon skip.

Yeah... a real pity. I really liked kmail but akonadi's tendency (at least at the time) to consume all available CPU and memory was too much to take and I moved to T-bird.

Coreutils!
The only reason I've used any FM was to quickly launch a whatever PDF reader available on a given host. Would be nice to have a standard command to show an association for a given file type or file.

I wonder. Perhaps it wouldn't be too difficult to write one. For example, you could have a script that used "echo `file $1`", then grepped for standard words in the output, like PNG or PDF and exec'd a corresponding program on the filename. More elegant would be a program that used libmagic directly to do the same thing.

The nice thing about it is you could start it off with a few options and a default ("No viewer for this file") and add types as you find them.

I vote for thunar.

Huh.
Why not just "file $1"?

It is easy to find a magic type. The whole point is to get the association (used by an FM, of course), which is located in different formats and different locations depending on the desktop software.

I was thinking of setting an association explicitly rather than using a mime database.

The MIME database does not contain names of associated applications and I really don't know of any universal method of finding an association. As for setting an association explicitly, yes, it will work of course but it will be very hard to maintain on different systems when associated applications change. That's exactly why I am using FMs to open associated programs even if it is slightly annoying each time.

PCManFM for old/very low-end computers, Caja on new computers without KDE, and Dolphin on KDE. My favorite is Dolphin, but all the dependencies aren't worth it off KDE.

Thunar works for me. Of course, that's what you'd want to use with an XFCE system, anyway.
Not this year's favorite (actually it's been unsupported for years although Pantheon is a fork of it) was Marlin, years ago. Since I don't run Elementary, I haven't used Pantheon though, so I have no idea. I hear you can use it in addition to your default file manager, but really, WHY?

Wouldn't have expected Dolphin to win this. Didn't think it was really that popular outside of people running plasma.