Using image alt tags for SEO

What are the Google guidelines for Alt Tags?

Google guidelines state that:

'When writing alt text, focus on creating useful, information-rich content that uses keywords appropriately and is in context of the content of the page. Avoid filling alt attributes with keywords (also known as keyword stuffing) as it results in a negative user experience and may cause your site to be seen as spam.'

So the primary purpose of the alt tag or alt text is to provide a partially sighted or blind person with a way of determining what an image on the page is showing. It should therefore describe what the image is. There is also an opportunity here to add a keyword, particularly if the image is especially relevant to that keyword; for example a product image.

So not filling in the alt tag or alt text is an opportunity missed.

Finding missing alt tags for images.

When building a website, alt tags are often the last thing one thinks about. Finding an appropriate image and then optimising that image for speed is my usual focus and it is very easy to forget that the alt tag is missing. It is also not immediately apparent when looking at the published site that there is anything wrong with the web page. So how do you know when an alt tag is missing?

I use tools, in particular Web Site Auditor which tells me in a content audit that there are images on a page that are missing alt tags. If you don't subscribe to the Web Site Auditor toolset then a free option is to use SEOoptimer's Alt Tag checker tool here.

Fixing missing Alt Tags.

Once found, it should be a simple job of finding the relevant image and adding a tag that is both relevant to the image and contains a keyword but does not stuff the tag full of keywords (which Google may consider spammy).

The alt tags exist at an image level and are attached specifically to one image and on a website should be unique to that image. They are usually coded at a programmatic level where images are presented on a website, because if the image is presented properly, there may be three variants of the image loaded for different screen widths. The website developer should present images which are optimised for the web (in webp format currently) and sized appropriately for desktop, tablet and mobile devices.

If the images are loaded from a content management system, then it is less likely that there will be three versions of that image, one for each device type. In this case then adding a caption to the image should satisfy the conditions for an alt tag to be added.

N.B. Although Alt tags are considered to be metadata (data about the image being shown) this should not be confused with the overall page metadata such as meta-title and meta-description and need to be set up separately. There is no magic switch anywhere where alt tags can be automatically generated/switched on. For a site with hundreds of images, it can be a manually intensive task to create these tags.