People who are blind do not access images directly. They listen to their screen readers read the alternative text that the developer or content creator provided for the image. In the case of users who are deafblind, the text is converted to refreshable braille characters that they feel with their fingers.
The techniques for supplying alternative text vary, depending on whether the image is presented in the image element, via SVG markup, via the canvas element, or via some other method. But the concept is the same: the alternative text must be meaningful and must serve as an effective substitute for the image in a way that makes sense to users who are blind.