Some people are experiencing problems when they try to post an image. However, just above the text box where you type a reply, are some icons. The third one from the end on the right hand side says, "Insert Image" when you hover your cursor over it. If you click that, optimally you should be presented with two options. One would be to select files from your computer. The second is to post an image directly from a url on the web. If you are choosing the latter option, make sure to click off the Retrieve remote file and reference locally option.
Good luck!