Can't find the tutorial thread, but I use Tumblr -- post a pic (or save it as a draft) > right click > "copy image address"
Or copy image address if it's any old picture on the internet already
Thank yu guys :)on the right side of the screen you can see your profile pic, click on it, there you can see a menu with different info like how many points you´ve got etc and under it is a menu - click on "signature" and here you can edit your signature the same way you edit your posts.
if you want to add a picture just click on "insert image" next to the table icon which is next to the smile emoji.
you can put there max 3 images