Screen readers can't "read" images. They can read alternative text (alt text / alt tag) if it is included in the image tag.
What is important to include in alt text? Several accessibility sites have suggestions since alt texts aren't an exact science.
Some browser's show alt tags when you hover over images on a web page. However, there are too many variables between operating systems and browers to make that process a consistently good test. These steps will work in all situations.
- In the input area after "Enter the URL of the web site you wish to evaluate" type the website address
- Press Enter.
- To find missing alt text, look for the red icon images or search for "Missing alt text."
- Even if all images have alt tags, read the alt text that's between the asterisks next to the image. Determine if the text conveys the information contained in the image.