Jpeg XL is a new-ish modern image format (ISO standard fully finalized in 2022), with compression rivaling or surpassing current formats. Reference encoder/decoder is libjxl in C++, and decoder jxl-rs in Rust.
It is royalty-free, a free and open standard.

Awareness and usage of it is about to increase in a big way, as support will be enabled by default in Firefox and Chromium within a month. And it is already supported by a wide variety of image-related programs such as Photoshop and Krita etc, and has been announced to be included in the PDF spec in the future.

With support for it I could, losslessly, cut the total filesize of my slideshow wallpaper library that's currently .pngs in half.

The official extension is for Win11 and I am on Win10 so I can't install that, but it is none the less working! Thank you!
I do have a 3rd-party jxl thumbnails thing installed that I am guessing might do similar hooks/support as the official one does: https://github.com/selpats/jxl-winthumb

I did get one issue that might not be directly related, applying wallpaper settings have gotten far slower this version.
Downgraded to 12.1.1 to compare, only using png images to be consistent across the versions, and it goes from under 10 seconds in 12.1.1 to about a minute and half in 12.2 beta 2.
The reason seems to be added overhead per image loaded. I have currently 134 images for my slideshow, and if I instead use single+single instead of single+slideshow on my two monitors it loads in <10 seconds, so the raw number of images seems to be the culprit where it wasn't an issue before.

With 5/10/20/40 images in the folder it takes ~10/12/19/32 seconds respectively. Plotting that with the previous 134->90seconds it looks to be pretty bang on a linear increase

Sure thing. Clicked on ok to apply wallpaper settings at about 19:02:34

With beta 5 the time to apply wallpaper settings is back to under 10 seconds, nice
The tray icon doesn't appear though (kinda like it, wouldn't mind having it as an option rather than bug), but no need to address that in this thread

The debug log? Sure, clicked apply at about 19:41:13, same subset of wallpapers as last time

So invisible vs actually gone? It's fully gone. Had the thought to check the taskbar settings and it was in the list turned off (curously still listed as DisplayFusion beta 4 which I'm not on), but enabling it didn't change anything as far as I can tell

I do not, it's the only entry and it's way down nearly at the bottom of the list

That did fix it. Tray icon is being displayed and listed correctly as Beta 5, no remnant of the Beta 4 version either