Free....? What is your shop worth to you? If it's free the banner may not turn out that good. You never know though...someone may be willing. Hey, your banner price you pay is a business expense Bananamoo! LOL : )
I sent a thread asking for help with my banner and a young mother answered and said she would help. After many convos and I wanted her to use my background, she now helped me have a more professionally looking banner.
She had Photoshop and the time and I had neither. Here is a link to her shop and she gave me permission to give her name out for others wanting help with their banners. I would consider paying a price because you and your shop is worth it!
Her name is Amber and I was glad to help a young mom out.
http://www.etsy.com/shop/ThekawaiiOD