Try this:
{$config.BASE_URL}/forums/images/{$style_array.general}/header.jpg
That would give you something like this:
http://www.yourdomainname.com/forums/images/general/header.jpg
As you have it in your current header it is calling:
http://www.yourdomainname.com/images/general/header.jpg
What is is actually outputting is
In other words, it is NOT parsing the $config.BASE_URL nor the $style_array.general parts at all. If I can figure out how to get it to parse correctly, I'm sure I can get it to work after that.