[Solved] How to add a picture here
I'm sorry, but wpForo plugin doesn't have the option to change the image you've mentioned.
I'll ask wpForo developers if it's possible to change via some CSS code or so. I'll back to you know asap.
I've asked wpForo developers this question I'm sorry, but there is no easy solution I can post here.
We've already added in our to-do list the option allowing to upload a cover picture. We'll add this in near future releases (no ETA).