When you are typing a reply or a new post, just below the text box is a panel labeled "Attachments". Click on the Browse button on the right, and locate the picture you want to post. Double-click on it and then click the Upload button. When the upload is complete, click on the pull-down menu labeled "Manage Current Attachments". On the far-left of the available options is a green + symbol. Click that.
Note: No matter where the cursor is in your reply, it will insert the link to the uploaded picture at the beginning of the post. You can move it to wherever you want it within the body of the message.