I think the tool is incorrect in this case. The alt attribute for most of the images (the avatars) is there, but it is set to blank. Blank alt attributes are perfectly valid HTML, as explained here:
The alt attribute is the text equivalent of the image, which means that it should only be empty if the meaning of the image is already encapsulated by the surrounding text.
If we put the username as the alt text for example, screen readers would read the username twice because it's next to the avatar.
The only exception might be the logo.png image, which appears to be part of the theme not Q2A. You can check on this site that the logo does have the site name in the alt. So you should ask the theme developer to make sure the logo has an alt text.