It's done using [ img ] [ /img ]
but you have to do it without the spaces.
Or you can just click the Img button above where you type stuff in.
You add the html image link inbetween the two img code brackets.
E.g https://image.spreadshirtmedia.com/image-server/v1/mp/designs/12386804,width=178,height=178,version=1370443645/winking-smiley-face.png
placed between the aforementioned code will produce the following image. Simply find a link to whatever image you want and then add it to your signature.