Describe, don't interpret

The most consistent request from blind and low-vision readers is to be given the same raw material a sighted reader gets, and left to draw their own conclusions.

The rules that follow from that

• Do not open with "image of" or "photo of". The screen reader has already said it is an image.
• Describe what is visible. Do not state someone's race, gender, age, or emotion as fact when you are reading it off a face.
• Do not sanitise. If the illustration is violent or unhappy, the reader is entitled to know that.
• Mention colour only when it carries meaning: a red warning line, a blue team, a green shoot in a bare field.
• Never write a number, name, or date you cannot actually read in the image or its caption. An invented figure is worse than no figure.

Length

One sentence for a simple image, ideally under about 125 characters. If it needs more than that, it usually needs an extended description instead, and a short alt naming what it is.

If there is a caption

The reader hears both. Do not repeat the caption word for word; describe what the caption does not already say.