If you don't add the image to your question because it is not relevant for the question itself, you can start writing an answer, and add the image to the answer using the post toolbar. You copy the link to the uploaded image, and use it to write your comment, writing the link as
[screenshot](http://i.stack.imgur.com/lXfJu.png), for example. After you copied the link, you can discard the answer you wrote using the "discard" link. The link doesn't appear immediately; if you don't see it, wait some seconds, and it will appear.
As suggested by Yannis Rizos, I would rather write that in the question, as it is relevant for everybody who answers your question. Clearly, I would not write "I tried what @username suggested, and it doesn't work for me" for every user who answers the question, but I would add a note in the question if I can give more information that can be useful to who answers the question.