Saved drafts are listed in the "user control panel" (see link near top left corner of the page).
Drafts can be edited and posted by going to "user control panel" then click on the "manage drafts" tab on the left side of the page.
Look for the "Saved at" column, then click "load draft". That will take you back to the message edit box in the original topic and you are then able to post by clicking the "Submit" button.
I know that it is not very intuitive, and it's not how I would have designed it, but we didn't write the forum software.
(Also note that saved drafts only include the subject and the message, other elements such as picture attachments will be removed.