How to Write Image Alt Text

Definition

Image alt text is alternative text for images for visually impaired users. It also serves as a signal for SEO and LLM image meaning recognition.

Summary

Alt text (alt attribute) is the alt property of the HTML <img> tag, read aloud by screen readers for visually impaired users. It is a core requirement of WCAG 2.1 Success Criterion 1.1.1 (Non-text Content) and is also used for Google Image Search optimization and LLM image meaning recognition. Use alt="" (empty string) for decorative images so screen readers skip them.

Problems This Guide Solves

This guide answers "how to write image alt text to achieve both SEO and accessibility." Many sites either leave alt empty or overstuff keywords—two extremes. Proper alt text achieves both goals simultaneously.

4 Roles of Alt Text

1. Accessibility (Most Important)

WCAG 2.1 Success Criterion 1.1.1 requires text alternatives for all non-text content. Screen reader users cannot see images, so they receive image information through alt text. Without the alt attribute, screen readers read the filename (e.g., "img_1234.jpg"), delivering meaningless information.

2. SEO

Google cannot visually recognize images and understands image content through alt text, surrounding text, and filenames. Clear alt text increases Google Image Search visibility and strengthens overall page topic signals.

3. AEO/GEO (Multimodal AI Era)

Multimodal AI models like GPT-4V, Claude, and Gemini can analyze images directly, but alt text remains a core signal for image meaning. Alt text also forms the basis for image citations in multimodal answer environments like Perplexity (see [Perplexity Citation Optimization] for details).

4. Fallback When Images Fail to Load

When images fail to load, alt text displays as text in the image area. This lets users understand page content even without images.

6 Rules for Writing Alt Text

1. Accurately Describe Image Content (Objectively)

Describe what is actually in the image. Avoid subjective interpretation or emotional expression. Describe objectively and specifically so someone who cannot see the image understands the content.

2. Length: Within 125 Characters

Screen readers typically handle up to about 125 characters smoothly. Include key information in complete sentences within this range. For complex infographics, summarize in alt text and provide full content in body text.

3. One Natural Keyword

You may naturally include the page target keyword once. Do not force it if it does not fit the image description. Use only when the keyword connects naturally to image content.

4. Avoid "Image" or "Photo"

Screen readers already announce "Image: [alt text]," so prefixing alt text with "image," "photo," or "picture" is redundant. "Line graph showing 40% monthly traffic increase after SEO optimization" is correct instead of "SEO traffic increase graph image."

5. Decorative Images Use alt="" (Empty String)

Pure decorative images for page design use alt="" (empty string). This makes screen readers skip the image. Omitting the alt attribute causes screen readers to read the filename, so an empty string must be explicitly set.

6. Consider Context

The same image may need different alt text depending on purpose. The same dog photo might be "Labrador Retriever's characteristic golden coat" on a breed introduction page and "happy dog playing outdoors" in a pet welfare article.

Good vs. Bad Alt Text

Good examples:

Bad examples:

4 Steps to Write Alt Text

Step 1: Classify Image Purpose

Classify the image into one of four types:

• Informational image: Meaningful alt text required
• Decorative image: Use alt=""
• Link/button image: Describe link destination or button function in alt
• Complex image (infographic, chart): Key summary in alt + full content in body text

Step 2: Identify Key Elements

Identify the core information the image conveys. For charts, key data points; for portraits, who and context; for products, product name and main features.

Step 3: Write in One Sentence (50–125 Characters)

Write key elements as a complete sentence in the 50–125 character range. Noun phrase lists are possible, but complete sentences feel more natural for screen reader UX.

Step 4: Validate by Reading Alt Without Seeing the Image

Verify that reading only the alt text (without seeing the image) provides sufficient understanding. Revise alt text if understanding is difficult.

Guides by Special Image Type

Charts/graphs: Include key data points. Example: "Bar chart showing AI search traffic increased 47% from 2024 to 2025"

Screenshots: Include what the screenshot shows and key information. Example: "Google Search Console Performance tab screenshot showing average CTR of 3.2%"

Portrait photos: Include who and context. Example: "ALLEO founder Hong Gil-dong teaching at an SEO workshop"

Product images: Include product name and main features. Example: "ALLEO dashboard screen with AI Visibility Score chart and keyword analysis section"

Infographics: Include only key summary in alt; provide full content in body text. Example: "GEO optimization 5-step process infographic (see body for details)"

Alt Text in the AEO/GEO Era

Multimodal AI models can analyze images directly, but alt text remains important as an explicit signal for conveying image meaning. Specifically:

• When AI processes pages, alt text is a primary signal for quickly understanding image content
• When images are cited in multimodal AI answers, alt text may serve as the image description
• Clear alt text related to page topic strengthens overall page topic consistency

Application in the Korean Market

Legal Requirements

Korea's Act on Prohibition of Discrimination against Persons with Disabilities mandates compliance with web content accessibility standards. Korean Web Content Accessibility Guidelines (KWCAG 2.1) also require image alternative text, consistent with WCAG 2.1.

Web Accessibility Certification

Image alt text is a core evaluation item in web accessibility quality certification audits by the National Information Society Agency (NIA). Public institution sites require mandatory certification.

Alt Input Methods by CMS

• Cafe24: Enter in "Alternative text" field after image upload
• Imweb: Alt text input popup when inserting images
• WordPress: "Alternative text" field in Media Library
• Naver Blog: Alternative text in image properties (see [Naver Search Advisor] for details)

Korean alt text is natural on Korean pages. Proper nouns such as brand and product names may remain in their original language.

Frequently Asked Questions

Q. Does every image need alt text?
A. Every <img> tag must have an alt attribute. Informational images use meaningful text; decorative images use alt="" (empty string). Without the alt attribute, screen readers read the filename.

Q. Isn't putting multiple keywords in alt good for SEO?
A. Keyword stuffing violates Google guidelines and has no real effect. One naturally placed related keyword in an image description is sufficient. Excessive keyword insertion also harms accessibility.

Q. Should I write in Korean or English?
A. Write in the page language. Korean alt is natural on Korean pages and provides better screen reader UX. Proper nouns such as brand and product names may remain in their original language.

Q. Do image filenames also affect SEO?
A. Yes. Meaningful filenames like "google-search-console-ctr-chart.jpg" instead of "img_1234.jpg" help Google understand image content. Consistent alt text and filenames reflecting image content favor image search visibility.

Sources

• W3C WAI. Alt text decision tree. https://www.w3.org/WAI/tutorials/images/decision-tree/
• Google Search Central. Google Images SEO best practices. https://developers.google.com/search/docs/appearance/google-images
• WCAG 2.1 Success Criterion 1.1.1 Non-text Content. https://www.w3.org/TR/WCAG21/#non-text-content
• Korean Web Content Accessibility Guidelines (KWCAG) 2.1. https://www.wah.or.kr/board/boardView.asp?brd_sn=5&brd_idx=853