At the bottom of the "reply" box, click "Go Advanced". Enter your text in the big box, then scroll down to the "Additional Options" area, and select "Manage Attachments". At the top click "Add Files", which will open your computer folders where you select the image file that you want to attach - click it and select "Open". That will list the file at the bottom of the Manage Attachments box, so you just select "Upload" and the picture will appear in your thread.