I have a great suggestion for you, if you need a professional site then you must hire a developing company, which will takes bucks but gives you a professional site like, printingray.com, printingblue.com, vistaprint, zazzle etc. These kind of sites are fully professional and have great value in search engines. Customers also prefer this kind of site because site represent the work. Best of luck
There are other places. This one does ordinary t-shirts too and will answer any of the questions you have. (However, to use the background colour of the t-shirt I would save the graphic as a TGA file with an Alpha channel).