You can make a banner from anything. When I first began, I would just compile clipart from a bunch of different sources. You can use something as simple as MS Paint, or use the free application of GIMP.
The banner on my forum was made using Photoshop. My PWS site was made with GIMP with a background image I personally took.
It takes a long time for a forum to take off...especially a weather forum. What I would do is find a few local folks that are interested in discussing your topics. Every now and then, others will filter in.
I'm proud because when you google "Gulf Coast Weather", we're the first link to pop up, above The Weather Channel
That took a few years to accomplish though.