1.You can find the images you like, then click and save it to the local disk.
2.Open the image with Photoshop, and choose the cutting tool and cut the size to 320*480，drag the potion you like with the mouse, then press "enter" to finish it. Choose menu-image-image size-set the width and height to 320*480，then change the format from web to jpeg.
3.Use iTunes upload the images to iPhone(iTunes download tutorial)，and set it.(tutorial)