Alt text needs to describe what's genuinely in an image, which means Claude is most useful here when it can actually see the image rather than guess from a filename alone.
Upload the Image, Don't Just Describe It
Claude can generate far more accurate alt text when it's given the actual image to look at, rather than trying to infer content from a vague description or file name.
Ask for Concise, Literal Descriptions
Good alt text describes what's visually present in plain language; steer Claude away from marketing language or unnecessary keyword insertion that doesn't serve accessibility.
Batch by Page, Not All at Once
Working through images page by page, with the surrounding content included in the prompt, produces alt text that's contextually relevant rather than generic.
Next step: Use the Page Size Checker to check whether the images you're writing alt text for are also worth compressing.