Can someone please tell me how to install GThumb 3.12 in Mint 20? this doesn't work, because it only installs version 3.8.

EDIT: Also, how does the AVIF algorithm work? I can't find any information about it at all.

Gthumb isn't listed in the Mint repositories, so they're likely using Ubuntu's leftovers. But seeing as your Mint is two major versions out of date which is falling out of it's support window, let's see: That's based on Focal, so...Latest package was 3:3.8.0-2.

That's it, thanks for playing.

This is why JWZ yelled at Debian, you know.

Fedora has 3.12.6, but I have no idea why you'd pick Gthumb over Viewnoir, Feh, or Ristretto.

Then again, I do outright and openly loathe everything Gnome stands for as a desktop paradigm.

Addendum: As for AVIF: Try reading this technical document.

Edit: Instead of the slightly snide remark, here instead is a logical screed against LTS:
Long Term Service is not meant for the you or I. It's meant for the computer that sits in the corner and maybe prints out something, like at a mechanic's shop where an onsite tech is out of the question and the budget. Where maybe it gets updated once a month if the stars align. I don't care what lies Mint or Ubuntu tried to tell you, it isn't made for use by the commons. If you want fresh and up to date software, you go with the flow of updates. Don't worry, if your computer was made within the last 20 years, it'll (probably) be fine, unless you're trying to cram in a Intel Titan or whatever passes for a modern graphics card alongside.

From what I can see, the PPA which is linked in the tutorial does include gthumb 3.12 for multiple targets:
(# = my comments, translating target codenames into versions and their support status)

gthumb
3:3.12.5-0jammy1    # = Ubuntu 22.04 LTS
3:3.12.5-0focal1    # = Ubuntu 20.04 LTS
3:3.12.2-0bionic1    # = Ubuntu 18.04 LTS - ESM - Expanded Security Maintenance
3:3.12.5-0mantic1    # = Ubuntu 23.10 -- EOL - End of Life!
3:3.12.4-0build2~ubuntu23.04    # = Ubuntu 23.04 -- EOL !
3:3.12.2-0impish2    # = Ubuntu 21.10 -- EOL !
3:3.12.0-0hirsute1    # = Ubuntu 21.04 -- EOL !
3:3.11.3-0groovy1    # = Ubuntu 20.10 -- EOL !

However, the PPA is for Ubuntu. And you are using Linux Mint. This is a Ubuntu derivative, but not the exact same. Does Mint even identify as one of those targets? I doubt it ... (but I'm no Mint user so correct me if I'm wrong).
Mint probably looks into this PPA and says: "Nah, nothing for me there." Which is why you still get gthumb from the official sources.

EDIT:
Maybe a general word of WARNING:
Only consider using PPAs (Personal Package Archives) if you really trust the source/ maintainer. This is 100% user-generated content without any checks and safety nets. And a years-old online blog / tutorial usually isn't a trusted source.
Importing PPAs and installing software from them is the Linux equivalent of downloading a random Windows executable from the internet. Maybe it is what you think it is, but maybe not. Nothing more joyful than executing some malicious script / trojan / botnet with your local admin or sudo privileges.

Thanks. Well now I've upgraded to 3.12.5 and I still can't view AVIF files, even they're supposed to work by 3.12. What's wrong?

Not sure why you did not just upgrade Mint but maybe your hardware cannot support that.

The tutorial says:

"With libheif library, gThumb now opens AVIF, HEIF (include Apple’s HEIC) images, and allows to save images as AVIF."

Do you have that library installed, too? I assume it works like a plugin. No library -- no AVIF playback.

It's less that it isn't compatible, but rather that they're not going to bother packaging it back to version 20, because who knows what may have changed in the interim.

You've got a whole chain of dependencies from high level libraries such as GCC down to mundane little things like your AVIF library there, which themselves may chain onto other utilities such as FFMPEG.

It'd be simpler to upgrade to Mint 22 (if they managed to make the upgrade process more seamless than the last time I used it). Which apparently, they've got this thing called MintUpgrade now. Huh, give it a shot. Though you may or may not (I wouldn't know) want to upgrade to 21 first.

Big note: If you decide to upgrade, disable all PPAs first. Last I used a Debian based system, it's dependency resolver left something to be desired, and unlike the AUR or COPR, the lack of centralization means that PPAs typically are single target.

SMPlayer should run that format. Assuming you just want it to watch yer stuff.

g2222, I already installed libheif though.

dnovraD, I don't know that I want to go upgrading the whole OS. Who knows what all would be changed?

Smacky-McNuts, I just got SMPlayer and installed it, but it won't display AVIF, or even normal formats like JPG.

"Who knows what all would be changed."

As a user of Fedora since version 25 or so? Not a whole lot to the common user.

Gnome in spite of itself has remained mostly the same since they changed the versioning scheme, KDE coalesced into it's current form around KDE 4, XFCE4 is the same as it's always been, and the nice thing is, there's desktops and WMs if you want to relive the old days like OpenBox, or WindowMaker.

Firefox is still Firefox, run0 is still run0, and release notes.