During the latest Webmaster Hangout, Google Search’s John Mueller answered three questions about image alt text (alt) and how it affects SEO.
Do I need to fill in alt?
Yes. Alternative text is useful for users who use screen readers, Muller says, and helps find images in image searches. However, alt does not affect page ranking in search.
Therefore, when writing alternative text, you need to focus on how to make the text understandable, not optimized.
Do I need to use alternative text if the image itself contains text?
Muller noted that it’s better not to use text in images, but noted that alt can be used in this case.
Should you use alt text when there is already text on the page that describes what is in the image?
The text on the page will be enough for search engines. However, it makes sense to use text for people who use screen readers. Muller also emphasized the importance of making alternative text informative. The text should describe what is in the image for people who cannot see it.
Source: Google Search Central