I think they have sophiscated software built-in to to resize images upwards and the actual dpi used for their printing is actually not 72 dpi.Originally Posted by Cheesecake
As for 1800 x 1200, it can't be 72dpi if your size is 4R because it would be 25" x 16.7" if it's printed at 72dpi. The dpi specified in the image filze may not be the one used for actual printing.
There is a difference between printing at home, printing in office and letting the commercial people print for you.
Anyway, here's a minimum image resolution guide for letting commercial people print for you : http://www.so-net.com.hk/imagestation/faq/Faq05.php