I have a blog over on Blogger at http://blog.adrianroselli.com/. I post regularly about trends and news in web development, usability, accessibility, social media, best practices, and anything else falls into the very broad category of "web related." Below is just the latest post from my blog, with links to more entries on the side. You can save yourself some hassle and just subscribe to the RSS feed and let it come to you.
Twitter's (current*) prohibition on retweeting oneself means that users cannot easily quote their own tweets to add alternative text — at least not in the Android app nor on the web. TweetDeck allows it, so perhaps we'll see it in the app or web site.
Because a quoted tweet is not a reply, it doesn't show up in the list of replies when viewing the original image tweet. This means a missed opportunity to be associated with the alternative text when viewed on its own.
When viewing the tweet providing the alternative text (as a standalone tweet, not in the timeline) in the Android app, the image within the quoted tweet is not displayed.
Embedding a tweet that acts as alternative text doesn't show the original quoted tweet (nor its image). There isn't an option to embed media, so web page authors will likely embed the original image tweet instead of the alternative text tweet. You can see this example below.
Twitter is missing an opportunity to provide an interface that prompts users to provide alternative text. This might help stem the inconsistent efforts from those who want to provide accessibility (such as 18F's noble but under-informed efforts).
* From the Twitter page explaining the feature, Note: You cannot Retweet your own quote Tweet.