Describe what is in the image and what the page is about. You get three alt text options under 125 characters, written for screen readers first and search second.
Describe the image in plain words: who or what is in it, what is happening, any text it shows. Add the page context and the keyword if there is one that fits, then choose the image type. Press Generate and you get three options with character counts. Paste your pick into the alt attribute of the image in your CMS. For decorative images, leave the alt empty.
Alt text is what a screen reader says when it reaches an image, and it is the main way Google understands what a picture shows. Missing or lazy alt text costs accessibility, fails audits, and gives up image search traffic. Good alt text describes the image accurately in one line, which happens to be exactly what Google wants too.
Under 125 characters. Some screen readers stop reading around there, and a description longer than that is usually padding.
Only when the keyword describes the image. Alt text stuffed with keywords hurts the people who rely on it, and Google treats it as spam.
An empty alt attribute, alt="". That tells screen readers to skip it. Leaving the attribute out entirely makes some readers announce the file name instead.