I know a few mural artists in America that can make your garage door into a work of art. I met them through the International Decorative Artisans (IDAL). For everyone one else, there is style your garage door.com
They have a large range of printed tarpaulines to choose from with instruction on how to affix them to an up and over style garage door. They can create one from your own design.
They ship worldwide.
I found them at Di Overton's wonderful blog Designers Block. Di is showing a sneak preview of her new work at the moment and has invited readers to help with her research. Take a look... it's fun.