The designs in the Marketplace belong to Spoonflower users just like you. All users retain the rights to their designs so we cannot make changes to them. If you would like a design slightly different, send a message to the designer through their profile page. Most designers would be happy to answer your request.
If you don't want to create a design yourself, graphic designers in your area or art students at your local colleges and universities would be great sources of expertise. Don't forget, lots of awesome designs have been created using scanned images made by hand or with photographs.