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What is Image Alt tag and Is It Important for SEO?

One of the most common SEO mistakes I see on websites is the absence of Alt tags on images. I suppose that many webmasters underestimate the importance of the Alt tag because they don’t understand its role in SEO.

In this article, I’ll explain what’s the function of the Alt tag in Search Engine Optimization and why it’s important to use it.

What is Image Alt Tag?

<Alt> is an image-specific attribute, which is added to the HTML tag of an image (or the <IMG> tag) and it’s is visible in the source code of the page.

The ALT attribute is designed to be an alternative text description for images. According to the definition given by W3C:

the alt text specifies an alternate text for user agents that cannot display images.


What is it used for?

Normally website visitors are able to see and interpret images simply by looking at them, so they don’t need the Alt tag to understand what the image is about. But there are other users that are unable to view the images, such as:

  • Users that have disabled images on their browser – some people prefer to disabled images in their browsers, so web pages can load faster. If that’s the case, your visitors won’t see the image, but they will be able to read the text in the alt tag and get an idea of what the image represents.
  • Users with slow internet that cannot download images.
  • Blind users that use screen readers to understand web content – the Alt text is read by screen readers, used by people with visual disabilities.
  • Search engine crawlers that cannot interpret graphic content -user agents such as, Googlebot and Bingbot can’t “see” and “read” images, so they use the text from the alt tag, to “understand” what the image is about.

Why is the Image Alt Tag important for SEO?

In a video from 2007, Matt Cutts explains that Google mainly focuses on alt text when trying to understand what an image is about. Google also said that:

providing useful, accurate information in your ALT attributes can make your photos and pictures more discoverable on the web.

In the same video, Matt Cutts explains that the <Alt> attribute is:

very useful for Google” and that it “is enough to help Google know what’s going on in an image”.

Matt Cutts

Therefore, when used properly, the alt tag text helps search engines to collect more information about your web page’s main topic and keywords. You can see it as an opportunity to increase the relevancy of the webpage for its targeted keywords.

How to write SEO optimized Image Alt tags

Now, when you know why alt tags are important for SEO, here’s a piece of advice on how to write a good image alt text:

  • Keywords: The alt attribute should describe what the image is about and should include the main keyword phrase of the page. If your targeted keyword doesn’t really match the picture, you may consider using another image, which is more adapted to the main topic (and keyword phrase) of the web page.
  • Length: I suggest limiting it to 10 – 15 words because if the alt text is too long, it won’t look good to users who have disabled images.
  • You also have the choice between writing an entire sentence or just a descriptive, keyword-rich text.

Why the absence of image Alt tags is an SEO mistake

In the beginning, I said that not including an “alt tag” to your image is a big SEO mistake.
I consider it a mistake, because, if you don’t use image alt

  • You’ve missed providing search engines with information about your image;
  • You’ve missed a keyword opportunity;
  • You provide poor user experience to visitors who have disabled images in their browsers.

That’s also why all SEO audit tools analyze the text of the alt attribute. They provide information about missing and duplicated alt tags – probably a negative signal for Google.

You will also find many SEO guides that list the image alt text as a main on-page SEO element.

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