Bobbie brings up a super awesome point. When in doubt (or lacking design skillz), ask the merchant or affiliate manager to hook you up.
As she said, many are only too happy to help because it means they'll also have some new creative for other affiliates as well.
If they can't or are unwilling to help, maybe you should reconsider actually promoting their affiliate program (assuming that your request is reasonable, of course).
~ Teli