An ALT attribute is a tag used within an <img> tag in HTML to describe to search engines what the image actually is. It is also useful for visually impaired users, who may be using a text-based browser, and users who have images disabled.
An example of an image tag with the ALT attribute set is below:
<img src="logo.png" alt="Image of company logo" >
To set these up depends on where your image is and how you have added it to the page:
This plug-in will automatically output the ALT attribute based on the group description.
This does not appear to output ALT attributes yet
Images in HTML Layouts (Templates)
Under Home > Layouts you can edit the HTML of the layouts to add in any missing ALT attributes to your images:
Don't forget to check-in and publish any changes you make.
Images in News Articles
Any images added via the HTML editor window (e.g. in News Articles or the CMS Editor) can have the properties updated by clicking on the image to select it, then clicking the Properties button:
This will open a new window allowing you to add the missing ALT attribute: