In the Reply to Thread window click on the down arrow next to the Paperclip symbol at the top of the reply window. The manage attachments windows opens. Under upload file from computer click Browse. The file upload window opens for your computer find the folder, file that you want to upload, then click upoad.
Then go back to message and click the down arrow again and the picture/file will be listed.
Click on it at a spot in the message where you want to insert picture.