One of my Stack Exchange communities is currently cleaning up its missing image alt text for the benefit of those who use the site with screen readers. We understand that the alt text we add will be offered in lieu of the image, and we know to omit it when it's totally redundant (image is pure decoration) and to focus on the description that the reader needs (e.g. "site logo" is more meaningful than a longer description of the logo).
We're a research-oriented site, and we've run into a few cases where the image is a scan from a book of a paragraph or so of text. (This is for cases where the text isn't available online, and sometimes where the book might be out of print.) We've talked about a few options in those cases, but none of us use screen readers nor know this field well, so we're coming to you.
We could retype the text and, while we're at it, just make that inline text (in a blockquote) instead of needing the image at all. Benefit: accessibility. Possible down-side: harder to skip over in cases where this was supplementary information (where sighted people might also just skip past the image).
We could retype the text as alt text, leaving the image for those who can see. We are not sure if that would be easier for those with screen readers than inline text is.
We could summarize the text and include a citation, e.g. "entry for (word) from (technical dictionary)", leaving it to the reader to find that source.
How should we approach this aspect of making our site accessible? What are users who rely on screen readers expecting from alt text in this situation?
Ours is a Stack Exchange site, so we are limited to Markdown.