Actually, when in doubt, hire a professional.
I have a lot of my work done by Lacey Wilson of Feral Design, and so do a few people here with their banner ads running. She does an excellent job. She also designed the flyer for FaunaClassifieds that I think you may have seen as well.
Feral Design
Normally Lacey will just forward the banner to me directly, and as long as payment has been made, I just set up the banner account and you are off and running. I would run it from September 1 thru October 4th for you. A few days either way isn't going to make much difference to me at all.
If you want to roll your own, then check out this page for the specs you will need to know:
Advertising Page
As long as your graphics program you are using will output in either JPG or GIF (which most of them will) you will simply save it in one of those two formats, then attach that file to an email to me, or attach and upload it here to a message and let me know about it. I will take it from there.
Any further questions?