When an image fails to load, or a reader cannot see it, the alt text is the image. It should carry the information the picture was there to provide.
Describe what the reader needs
If the image is a chart, say what the chart shows: the trend, the comparison, the number that matters. If it is a photo of a person at a desk, say who and what they are doing, not "image" or "photo".
Skip alt text on decoration
A background pattern or a divider does not need a description. An empty alt attribute tells assistive tools to ignore it. Inventing a caption for decoration adds noise.
Do not start with "image of"
The reader already knows it is an image. Start with the content: "Line chart showing reading time falling as the font size rises from 14 to 20 pixels."
Write the alt text when you choose the image, while you still know why you included it.