Sitecore provides a cleanup agent to clear older media files, which clears media files every specified interval of time. By default it clears all media cache files created 90 days ago. See below settings in sitecore.config:
<agent type="Sitecore.Tasks.CleanupAgent" method="Run" interval="06:00:00">
<!-- Specifies files to be cleaned up.
If rolling="true", [minCount] and [maxCount] will be ignored.
[minAge] and [maxAge] must be specified as [days.]hh:mm:ss. The default value
of [minAge] is 30 minutes.
[strategy]: number of files within hour, day, week, month, year
[recursive=true|false]: descend folders?
-->
<files hint="raw:AddCommand">
<remove folder="$(dataFolder)/logs" pattern="*log.*.txt" maxAge="30.00:00:00" />
<remove folder="$(dataFolder)/diagnostics" pattern="*.*" maxAge="30.00:00:00" recursive="true" />
<remove folder="$(dataFolder)/viewstate" pattern="*.txt" maxAge="2.00:00:00" recursive="true" />
<remove folder="$(tempFolder)/diagnostics" pattern="*.*" maxAge="00:10:00" recursive="true" />
<remove folder="/App_Data/MediaCache" pattern="*.*" maxAge="90.00:00:00" recursive="true" />
</files>
</agent>
It is not recommended to clear media cache very often because you will have some performance impact.
I would say this is expected behavior since the file is cached by the browser since it accessed the file previously. If you want to force the cache clear for the changed image you would need to change the url in your embedded html and add for example a modified value in the url, similar to the approach described Forcing Sitecore to clear the cache for updated images
