Joomla offers the possibility to mark an image as "decorative" (Decorative Image - no description required) and the alt attribute is correctly added as empty (alt="").
Reading recommendation
WCAG Success Criterion (SC) 1.1.1: https://www.w3.org/WAI/WCAG21/Understanding/non-text-content
Writing Alt Text for Data Visualization
Write helpful Alt Text to describe images