There could be a few reasons why the image is disappearing. The URL for the image is a unique temporary URL that is generated and managed by Moodle. The URL may change from one page load to the next, when enabling/disabling edit mode, when viewing the page as a different user, etc. You cannot copy it from one category and have it work in another. You cannot remove it, save the description, edit the page again and paste it back in. If you do anything like that, the image URL will fail. Maybe not immediately, but it will likely fail anywhere from within minutes to the next day.

In order to ensure that it will work reliably, you must follow the exact steps:

1) Upload the image into the editor for the category description.
2) Edit the HTML and paste in your new HTML code.
3) Cut and paste the URL from the original HTML code into the new HTML code.
4) Delete the original HTML.
5) Save.

If you deviate from this process, Moodle looses track of the temporary URL and will eventually fail.

I’ve done this on multiple recent versions of Moodle. If this does not work for you, please share the version of Moodle as well as the name of the theme that you are using.

Hope you find this helpful. Let me know how it goes.

Best regards,

Michael