Alt attributes of images for SEO: textual alternatives

What is an ALT attribute? 

Google doesn't know how to interpret your images. In order to allow Google to understand your content, you can write an explicative text for each image. The ALT attribute allows you to add an alternative description for images of an HTML page. It is defined in the code of your page:

<img src="photo.jpg" alt="picture of my holidays in Miami" />

How to optimize your ALT attributes for your SEO?

There is no standard length for the text of your ALT attributes. Avoid lists of keywords separated with commas and opt for a precise description of the content of your image. Ensure that the description matches with the content of your page. 

Example

Perfect ALT attribute: Malinois dog sat on a country road. 

How to know if my images have defined ALT attributes? 

Go in your page analysis. Open the content tab and check the Images criterion. You can find a table listing all the images of your pages and their textual description.