Another way you can attach a picture to your discussion post is to upload your picture on Imgur, get the Markdown Link for the picture (second from top link, on the right), then copy-paste it to your comment (add an exclamation point before the link, or else it will turn into a link [to people reading your comment in the forums, they can click on it and it will take them to your picture]). It's much easier for me to upload pictures this way.

The uploading screen on Imgur:

Source: this comment (You can find it yourself by going to Troubleshooting —> Top All Time —> and it's the fourth-highest comment [if I'm correct] there.)