Facebook rescans pages every 24-48 hours, so the new image will start working soon. The only other thing to know is that if you wanted a different image just for the forums, you can do that in the same area you added the {JFBCLike} tag by adding the following: {JFBCGraph image=http://site.com/image-for-forums.jpg}. That way, you can have a logo for your site set as the default for all other pages, and for forum pages, it'd be a different logo.. Not necessary, but just wanted to let you know it's possible.
Finally, you can always use the Facebook Debug tool to see what Facebook sees about your page. This is good for testing after you make Open Graph changes to make sure they're done properly, instead of waiting 2 days:
developers.facebook.com/tools/debug
Good luck,
Alex
How to control image posted to Facebook with Like in Kunena
