What good alt text actually says
Strong alt text describes what's in the image and why it matters to the page. Not 'a photo of a dog' — too vague. Not 'image1234.jpg' — useless. Better: 'Golden retriever mid-leap catching a frisbee at sunset on Santa Monica beach'.
The rule: if you removed the image, what sentence would the visitor need to understand the page? That sentence is your alt text. The generator analyzes the image content and outputs alt text at the right specificity level (75-125 characters is ideal — long enough to be specific, short enough that screen readers don't drone on).