Describe the purpose
What does the image convey here? A product photo needs the product; a decorative divider needs nothing at all and should carry an empty alt attribute.
Keep it short and specific
A sentence is usually enough. "Chart showing uptime rising from 97% to 99.9% after migration" beats both "chart" and a paragraph.
Do not stuff keywords
It reads badly aloud and search engines discount it. Alt text written for a person is the version that performs better anyway.