Once you get your Edit Signature option (15 posts plus about an hour's wait), it will be found by pressing Settings in the upper right of the forum, then look down the left side of the settings page to find Edit Signature.
What I had to do to get my
vaping days picture signature to work was paste the HTML for it into the Edit Signature text box, then carefully select exactly all of the HTML, then press the image icon above the text box (I think it may be next to an envelope icon). That adds a bit of image tag stuff to the start and end of the HTML. Then save.