Go under 'Profile' here on the forum, and go to 'Modify Forum Profile' by hovering over the buttons on your profile page. From there, copy the image link you're going to use and put it in the image code.
[.img.][./img.]
Paste the link between those two blocks. And before you save, remove the dots or the code won't work.
After that, scroll down and hit save.
Keep in mind that the forum doesn't allow the use of Tinypic, so try using imgur or other image hosters instead. <3
~ Whisper