http://img532.imageshack.us/img532/471/36198432.png
thats your image pasted into a 300dpi page and then saved and spreadshirt.com bounced to spreadshirt.co.uk for me and the t-shirt builder thing seemed to accept it.
ideally though printers seem to prefer vector images that can be resized with no quality loss
with anything like this though if you find someone that says they can do it make sure you get a proof done before you go ahead and order 500 or you could end up with 500 unuseable rags.