The useful starting point
A decorative scene and a chart carrying a result have different roles. Decide what the reader would miss if the image were unavailable. A caption may already explain some of the content, so the alternative should not simply duplicate a long paragraph.
A small step to try
Review the image together with the surrounding text. Describe the relevant information and keep fictional illustrations clearly separate from evidence. An abstract generated laptop scene is not a screenshot of a working service. Use W3C guidance to choose an appropriate treatment, then inspect the actual page. Good wording is part of accessibility work, not a complete assessment of the interface.