Joost Kremers writes:

> If I manually change the link to a relative one on my computer, Emacs (27.2)
> shows the linked png just fine (provided the png image is in the directory
> pointed to, of course). Why your Emacs isn't able to find the png, I don't know.
> Perhaps you moved it in between viewing the svg in Firefox and Emacs?

OP here, apologies: I neglected to make the link in my demo code easily
transferrable for testing!  I’ve streamlined the reproduction case.

Please try this:

$ emacs -Q demo.org

...where the org file is the one that’s attached, and the PNG file
Tft1st.png is saved in the /tmp directory:

$ ls -lt /tmp/Tft1st.png
-rw-rw-r-- 1 joe joe 391204 May 18 15:00 /tmp/Tft1st.png

Lastly, evaluate the Org block with C-c C-c.