I'm trying to create retina display wallpaper for my iPhone but every time I change the pixel count to the 640 X 960, it doesn't work. (e.g. a 769 X 1024 changes to 640 X 852 with a resolution of 72). My three questions are the following: 1. Do I need to go get photoshop or something? 2. Does having a image with a resolution over 640 X 960 already make it a retina display? 3. How can I use preview to resize my photos correctly? Thank you so much!