You can add your own customized banner on your townhall link.
The requested design is 1240x200px.
If you don't have the tool or the designer to create a banner, we recommend to do it on www.canva.com, a very easy and free design creation website.
Be careful to only use licence-free pictures and pictures which you own the rights.
Make sure you use high resolution materials.
Finally, we advise not to create a white banner because it will merge with the white background of the site.