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For various reasons, people need to use screen readers. To my knowledge, screen readers can't read text from images, so this is an accessibility issue. 

In addition, some people might just find it hard to read text in images.

While there's nothing wrong with using screenshots, it is important that the text you're quoting *is also* typed into the post.

With that context, when someone  says they're downvoting because it's an image, the (an?) underlying problem is because (other) people either struggle to or cannot read it.