High-Res Images

Hello, I have a script that process images sent by users, but I'm having some problems here. 1) Does anybody know if there is any problem with hi-res images (300 DPI), because when I try to use them with PHP to resize/copy them, all I get is a black box. 2) Is there any way, with GD/PHP to get the DPI value of an image, because the image they submit has to be 300 dpi. In short, I need the user to upload the file, and the script to verify it is 300 DPI and to resize it to its equivalent size with 72 DPI. ANY help would be really appreciated. Oscar F.-