Is it inappropriate to post long answers with sample illustrations?

Long answers, whether or not it's due to screenshots, are often a sign that you've been unnecessarily verbose. Usually, if it actually does take several screens worth of information to answer a question, the question was too broad in the first place. If a question is of reasonable scope, usually it's possible to answer it more concisely. Answers as long as yours are often less useful to readers, and a downvote on an answer means, as the hover text says, "this answer is not useful", so it shouldn't be too surprising to see a downvote. You should generally interpret it as motivation to be more concise, which is not necessarily the same thing as being less thorough.

In the specific case of your answer, the screenshots do indeed appear a bit iffy. Though you've numbered and referenced the screenshots, there's no way to jump back and forth easily between screenshots and text, and it's difficult to tell from the text which ones are "for reference" and which ones are actually worth the time to go look at. It's also unclear if the detailed steps to reproduce are really necessary in order to answer the question. Particularly if you've already posted it on your blog (though your link doesn't take me to anything useful) there's no need to also post it all in an answer - you can just provide the link for more information. So it seems that the length is in some sense unnecessary. But even if it is necessary, when an answer gets this long, it's likely worth putting in a bit of effort to shorten it where possible - avoid being too wordy or adding nonessential detail. At the end of the day, what really matters is how useful it is to someone who finds it later, and a lot of those people are going to want to know how to fix things without spending 15 minutes reading.