Alternative (Alt) Text is meant to convey the “why” of the image as it relates to the content of a document or webpage. It is read aloud to users by screen reader software, and it is indexed by search engines. It also displays on the page if the image fails to load, as in this example of a missing image.
Best Practices for Alt Text on your webpage or document
- Add alt text all non-decorative images.
- Keep it short and descriptive, like a tweet.
- Don’t include “image of” or “photo of”.
- Leave alt text blank if the image is purely decorative
- It's not necessary to add text in the Title field.
- Use one of the testing tools to hide images and display their alt text. Does the page make sense with the images hidden, replaced by their text alternatives
- Do all linked images meaningfully describe their action rather than their appearance?
The most appropriate alternative text for an image depends very much on the context of the image in question. You must provide information for that image that takes into account its purpose and also the surrounding text on the page. The same image might need different alternative text depending on how it's used.
Examples
Let's look at some examples of appropriate alternative text, using the same Harvard University logo in different contexts.
Stand-alone
In this basic example, we’re just using the Harvard logo to indicate that the page is associated with Harvard University. In this case, the alternative text should say something like "Harvard University logo."
But if the logo were provided as an example of logo design, we might want to go further and opt for something like "Harvard University logo, featuring black, serif text and a red shield emblem", as in this example:
Inside a link
It's a web-design convention to use a logo graphic as a link back to a site's home page. In this case, we would use alternative text that describes the action of the link. After placing the logo inside a link pointing to the home page, appropriate alternative text might be "Harvard University home page" or similar. A screen reader would identify that the image is also a link and say, "Harvard University homepage, link."
Inline with text
In rare occasions, an image of text might be used inline as a substitute for text. For instance, we might want to write "Welcome to Harvard University" using the logo instead of the words "Harvard University." In this case, we should focus on writing alternative text that stands in for the visible text within the logo, without adding any extraneous information—that is, you’d want the alternative text to just say "Harvard University". We wouldn't want a screen reader announcing, "Welcome to the Harvard University logo"!
Welcome to 
Examples of Alt text on a simple image
Alt-text with no context:
A mostly empty stadium.
Alt-text with context about the actions in the image:
Harvard Stadium with two lone runners bounding up the steps.
Alt-text with context on the setting in the image:
Harvard Stadium with cracked concrete pillars.
How to Write Alt Text for Complex Image Types
Icons
Alt text for icons should be the equivalent to the information intended by the icon, such as “Download PDF” or “Visit our Facebook Page”
Images as Links
If the image is being used to link to another page, the alt text should describe what will happen when the image is clicked (rather than what it looks like). For example, the alt text for an image of a question mark that links to a help page should be “Contact Support” rather than “question mark.”
Images with complex information
If the image is a chart, diagram, or illustration, consider how to convey the information contained in the image using both the alt-text and the adjacent page text. See an example of an accessible infographic from WebAIM.