Are image alt attributes truly critical for your Google SEO?

Why does Google emphasize alternative texts?

Google does not see images as we do. A crawler only perceives a binary file, and without textual signals, the algorithm cannot index or rank the image properly in Google Images. The alternative text provides this signal.

The caption plays a different role: it contextualizes the image within the reading flow. Google gives it weight because it indicates the editorial intent surrounding this visual. The two elements are not redundant; they complement each other.

What is the difference between alt text and captions in terms of SEO?

The alt attribute is read by screen readers and serves as a fallback if the image does not load. Google uses it to understand the subject of the image itself. The caption, visible to all, enriches the textual content of the page and reinforces the overall thematic relevance.

An alt must factually describe what the image represents. A caption can be more flexible: interpret, contextualize, guide the reader. Both are crawled, but their usage differs.

Does Google specify the quantified impact of this practice?

No. This is the recurring issue with such statements: no quantitative data accompanies the recommendation. We know it is useful, but not to what extent compared to other factors (text content, backlinks, EAT).

Field observations suggest that optimizing alt texts primarily enhances ranking in Google Images, with an indirect effect on overall SEO through additional traffic and time spent on the page.

• Alts are mandatory for accessibility and technical SEO.
• Captions enhance the semantic relevance of the entire page.
• Google Images generates qualified traffic if the images are well-optimized.
• Lack of alt harms crawling and penalizes the indexing of your visuals.
• No official figures on the direct ranking impact of alts.

Is this statement consistent with what we observe on the ground?

Yes, but partially. E-commerce sites that systematically optimize their alts notice a traffic increase via Google Images, often between 5 and 15% of total organic traffic depending on the sectors. It is measurable, reproducible.

However, the effect on the ranking of regular pages remains difficult to isolate. A well-written alt improves overall semantic coherence, but there is no proven direct correlation between alt density and SERP positions. [To be verified]: Google does not publish any quantitative case studies on this point.

What nuances should be added to this recommendation?

The first nuance: not all alts are created equal. A generic alt like "image1.jpg" or "product photo" is useless. Google expects a factual, contextual, and unique description. Copying and pasting the same alt across 50 images in a product gallery is pointless.

The second nuance: the caption is often overlooked. Yet, it can carry long-tail keywords that the alt should not overload. If the alt describes, the caption can interpret or enrich. Both need to be coherent without being redundant.

In what cases does this rule not fully apply?

Decorative images (icons, separators, CSS backgrounds) do not require a descriptive alt. An empty alt (alt="") is preferable to avoid polluting the screen reader experience. Google tolerates this practice if it is justified.

Dynamic images (graphs, interactive heatmaps) pose challenges: it's difficult to create a relevant alt if the content changes based on the user. There, the contextualizing caption becomes more important than the alt. If the image is complex (dense infographic), prefer a long HTML description over an overloaded alt.

What should you do concretely on an existing site?

First audit all images without alt using a crawler (Screaming Frog, Oncrawl, Sitebulb). Prioritize strategic pages: product sheets, landing pages, pillar articles. Decorative images can wait.

Next, write descriptive alts in 8-12 words maximum. Include the main keyword if natural, but never force it. Example: "Red and black trail shoes for men" instead of "Men's mountain running trail shoes performance".

What mistakes should be absolutely avoided?

Never reuse the same alt on multiple images. Each visual must have its unique alt. Google detects duplication and infers that the optimization is automated, thus unreliable.

Avoid overly long alts (> 125 characters). Screen readers truncate them, and Google may view it as an attempt to manipulate. Fact-based conciseness is paramount. If the image requires a long description, use the longdesc attribute or a detailed caption.

How can I check if my site is compliant?

Run a Lighthouse or WAVE audit to detect images without alts. Check also for semantic coherence between alts, captions, and surrounding content: an alt disconnected from the adjacent paragraph loses relevance.

Monitor Google Search Console, "Performance" section filtered on Images. If your Image traffic stagnates despite optimized alts, the problem lies elsewhere (file sizes, poorly configured lazy loading, missing image sitemap).

• Crawler the site to list all images without alts.
• Write unique alts for each strategic image (8-12 words max).
• Add contextualizing captions on important images.
• Avoid keyword stuffing in alts and captions.
• Leave alt="" for purely decorative images.
• Check semantic coherence between alts, captions, and text content.