Since upgrading to Kubuntu 26.04 LTS, images are no longer displayed

[Schwarzer Kater]

Schwarzer Kater I’ve now managed to follow your instructions successfully, but now my old images – the ones I created before the upgrade – are no longer visible either.

I am really sorry to hear that…

It seems something has been seriously broken by your do-release-upgrade -d from 25.10 to 26.04.

The final thing I (personally) would try is to create a new user account with administrator privileges ( -> System Settings -> Users (section: System) ) and test if the problem is still there. If no: transfer your data to the new user account and don't think about it anymore.
If yes - the problem also is there in a new user account: I (personally) would not waste any more time, but backup my data, erase the old system and start a fresh installation of Kubuntu 26.04…

[life0riley]

This is an upgrade I can see from the subject line.
Are you using Wayland or x11?
What is the output of:

echo $XDG_SESSION_TYPE

The output looks like this on mine:

$ echo $XDG_SESSION_TYPE
wayland
$​

There was a bug a year ago for x11 only.
https://bugs.kde.org/show_bug.cgi?id=503799

[greenster]

systemsettings -> default applications -> file associations what does it list for .png?

See here:

[Schwarzer Kater]

[…]
There was a bug a year ago for x11 only.
https://bugs.kde.org/show_bug.cgi?id=503799

Just my two EuroCents: I don't think this is an X11 problem.
I am using both X11 and Wayland (fresh 26.04 installation) and everything works everywhere regarding image previews - doesn't matter whether old images or images I just created…