Hi Jan,
I just tested the advanced description field for users, but it works with HTML tags on my end: https://i.imgur.com/qgQOsdM.png
Make sure you have the latest release installed (4.14.5) and that there are no unclosed tags there in the content. That should hopefully do the trick.
With a custom code it should be possible to get the user image from a custom field as well. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
Change the $field variable to the user meta field containing the image URL.