Not really very helpful this, as I can't remember where we got ours.
Wanted to say though, go for a heavy duty banner, PVC coated with eyelets, then you can tie it securely and it won't tear in the wind, or with use. Take strong cord.
You can choose a photo as your backing, or plain, or a series of smaller pics, but don't make it cluttered with too much going on. As a customer I would probably respond to a lovely pastoral scene if you're selling meat, or a beautiful, very clear veg garden if it's veg, with happy hens etc. Obviously make your name large and the main feature.
With the maker we chose, we were able to try out all the options on the 'puter to see which was most effective, before finalising.