I would DEFINITELY not do a wordpress.com hosted site. Worst idea ever. That's basically playing to lose.. because assuming you DO succeed all you've done is create a HUGE problem for yourself. You've got a following that's linked to a site hosted by someone else. Terrible. Don't start off with a hosted blog, come up with a domain name and purchase it for $11 a year, set up some cheap hosting.. $20 a month or so, install WordPress on the domain and start writing.
You can design a banner yourself in Photoshop or get someone from Elance or oDesk to do it for you.
You can design a banner yourself in Photoshop or get someone from Elance or oDesk to do it for you.