You could download Photoshop, it has a 30-day trial period. You could also browse the net for iPhone Winterboard themes and create an icon that just mixed together from different templates.
heres an idea, adobe illustrator. The main thing that sells a user to buy your app is the title and the icon because thats what they see in the app store on first glance. You need really good graphics if you want your app to be successful so i suggest simply investing some money in Illustrator.