Symptoms
Images in the Event Streams UI are rendered as a black-filled shape instead of the correct image as shown in the following example:
Causes
Mozilla Firefox 76 and earlier versions block the inline CSS styling in SVG images. This is because they incorrectly apply the page-level Content Security Policy against the SVG images.
Resolving the problem
Update your Firefox version to 77 or later as the issue has been resolved in Firefox 77.