How To attach a file
Use the "File attachments" option at the bottom of the page, just above the Save and Preview buttons when you're typing your post. Click on Choose a file
it is also allowed to attach a backup file (starting with a dot, ending with a comma).
Remember, after selecting the file, to use the Save button.
After attaching the file, in the case of an image, you can insert it in the message.
Move your cursor in your reply (or post) where you want the picture then click the insert link (the underlined blue word insert)