The resolution of your original image becomes almost a moot point with these people (bignose.com provides the prints), because the upload applet automatically applies jpeg compression. The file that gets printed ends up being terrible quality.
bignose.com also uses inkjet technology to make printouts, which still hasn't surpassed actual photographic processing.
After I saw my tiff image was reduced to a jpeg 1/10th the size of the original, I wasn't expecting much, and they didn't deliver much. (Keep in mind that I sell my pictures, so my print standards may be higher than others.)