Unable to get apache2 to serve changed image

I replaced two images linked from one of the pages on my intranet,
but the old versions continue to be served. Kind of.

I restarted the browser (FF on Ub 18.04), with no change. I restarted
apache2, with no change. I tried calling up the page on my tablet
(iPad running Safari), and continued to get the old version of the
images.

I made a new page, with no content other than links to the new
versions of the images. A pair of links to the new versions under
their proper names, the pair were to copies of the new versions. The
first pair of links still showed the old versions, the second showed
the new ones.

$ cp -p 202308dp.png stupiddp.png
$ cp -p 202308tr.png stupidtr.png
$ ls -Sl 202308* stup*
-r--r--r-- 1 username username 38501 Sep  1 08:08 202308tr.png
-r--r--r-- 1 username username 38501 Sep  1 08:08 stupidtr.png
-r--r--r-- 1 username username 13741 Sep  1 08:09 202308dp.png
-r--r--r-- 1 username username 13741 Sep  1 08:09 stupiddp.png
$

The 202308* files are the revised files that I want to show. The
stupid* files are, as is shown, exact copies, which do show. They
are identical, so the failure to display is not due to a problem
with the files.

Then, I went to a different computer, one that I was sure had never
called up the page in question. It properly showed the new version.
This also supports the idea that there is no problem with the files
themselves.

But, when I went back to the box where I was originally working, it
still showed the old images.

I tried adding the line:
EnableSendfile off
to my /etc/apache2/apache.conf
followed by restarting apache2, as suggested by:
<https://stackoverflow.com/questions/12472528/apache-caching-images>

I tried:
$ sudo a2enmod expires
followed by a restart, as suggested by:
<https://www.arubacloud.com/tutorial/how-to-optimize-the-cache-of-static-files-of-apache-on-ubuntu-18-04.aspx>

Neither of these had any effect.

It should be noted that I upgraded Ubuntu (xenial ==> bionic) since
my previous modification to the page with the problematic display.

What do I try next?

-- 
Michael F. Stemper
No animals were harmed in the composition of this message.