My partner and I saw this market opportunity a couple of months ago. For a first version we came up with 9 banner templates and instead of allowing the user to update the banners themselves we do it manually within 24 hours.

site: http://33banners.com/ blogpost: http://aflanagan.com/will-this-make-500-a-month/

From our experience I think your biggest challenge is going to be reaching potential customers.

Best of luck.