How Alt Text Generation AI Actually Works for Images

Alt text is the hidden description that appears when an image fails to load—and more importantly, it's what screen readers (software that reads web content aloud) read to people who are blind or have low vision. Without it, an image is completely invisible to them.

Here's what traditionally happens: a sighted person manually writes a description for every image. It's tedious, time-consuming, and often inconsistent. Some descriptions are too vague ("photo"), others are unnecessarily long.

How AI changes this: Modern AI models like Gemini or Claude can "look at" an image and understand what's in it—objects, text, actions, colors, context. When you run an image through these tools, they analyze visual patterns and generate descriptive text in seconds. The AI isn't actually seeing like you do; it's recognizing patterns in pixel data and converting those patterns into words.

Why this matters: An AI-generated alt text for a chart doesn't just say "graph." It might describe: "Bar chart showing disability employment rates from 2015-2023, with employment increasing from 18% to 23%." A screen reader user gets the same information a sighted person does from glancing at it.

The catch: AI-generated alt text needs human review. AI sometimes misses context (cultural references, emotional tone) or includes unnecessary details. Think of it as a first draft, not a finished product. Someone familiar with the content should always check it.

This is especially important for complex images—medical diagrams, data visualizations, logos with meaning. For simple decorative images, AI can confidently generate "decorative pattern" or flag them to be marked as decorative (so screen readers skip them entirely).

Real scenario: A nonprofit publishes an accessibility report with 47 charts and 120 photographs. Instead of hiring someone to write alt text for months, they run everything through Gemini, spend an afternoon reviewing and refining, and launch on schedule with fully accessible content.

The AI does the heavy lifting—pattern recognition and description writing—while humans do what we're best at: understanding context, catching errors, and making judgment calls.

Try this: Take a screenshot of a complex image (a chart, infographic, or photo). Paste it into Gemini or Claude with this prompt: "Describe this image in detail as if you were explaining it to someone who cannot see it. Include all text, numbers, and important visual elements." Compare the AI output to what a sighted person would actually notice. What did it miss? What was too detailed? This teaches you what makes alt text genuinely useful.