Screen readers. There are programs for the visually impaired that can read out specific parts of transcriptions for images, but it requires an adequate transcription to be in place.

My transcriptions follow the typical ToR format - first it identifies the type of image, then it provides a description of the image itself, giving the necessary contextual details to have a decent grasp on what they are looking at. Finally, it lists the text placed over the image. The different sections are separated from one another using paragraphs and line breaks, and labelled, so the screen reader can easily jump from one to the next with ease. This allows the person to have a decent idea of what the image was trying to convey, so they can then participate in the post and experience what the other sighted users are experiencing.

Transcriptions are also there for people who have internet connection issues and can't load the image, or who have a difficult time understanding what they're looking at and could benefit from a description of the visuals themselves. But the format is very intentional in order to be screen reader compatible, to allow blind and visually impaired users to participate in enjoying the post.

Hope this helps!