What exactly is a high-resolution image?
We require a tiff image that has been scanned at 300 dpi and will fill up the printable area of the print size you select. So, for example, for a 8.5 x 11 in print, the printable area is about 8 x 10, and your image needs to be 2400 x 3000 pixels. (This is the most popular size, but larger can always be shrunk down, and the opposite is not true.