I looked around a bit because I had a few issues with it as well.
If you click your name up in the right hand corner you should get a dropdown menu where there's a button to access your signature. You can post the link to the image there by clicking the image button, and then clicking the image and hyperlinking to the game thread.
However, it seems you have to make at least 3 comments or posts on the forums before it will let you do that. I haven't done that myself yet, so I'll be posting a few comments on some of the games I play