@jupiter_rowland Just a correction to the post I'm replying to. There are things called speech history. It is an addon for NVDA and from what I hear standard in Jaws. Those are the two largest and most used screen readers out there. This means that super long alt text isn't the seeming barrier that most think even though it is still technically corect that alt text isn't as navigable as normal text.
As far as what you should put into alt text. Here are the 3 resources I always point people to when that question gets asked. I don't really think that the alt text should be an essay about the picture, but it should contain enough information to get the picture's content across.