Help! Image resize not working !!! [Locked. Duplicate.]

I need to make a facebook cover ( 851 x 315 ,300 resolution rbg) I have a image that i have to use.The image is 4471x 2976 px ,300 dpi.When i place the image into the photoshop document that has the size for the cover , it loses resolution so much! I tried to resize it but stil nothing. Please help me!