Help - Cropping Photos

I'm trying to crop some wedding photos on Kodakgallery.com but the images are coming out blurry. I'm using the hi-res images my photographer gave me. The website is showing that the photo can print best up to 20' x 30' and the resolution is 3872 x 2592 pixels. I don't kow what I'm doing wrong Any help would be greatly appreciated!!

TIA!