Previewing Your Image:
The UI Preview shows you what your image looks like at various representations on different backgrounds. This is useful for previewing images for desktop applications, which Apple recommends that you test at 1.0x, 1.25x, and 1.5x resolutions. At the bottom of the Preview window you can add more resolutions to be previewed and also customize the background behind the preview images.
The Cursor Preview creates a cursor from your image. This cursor is shown against a background that you can just like for UI Previews. You can also set the hotspot (the point in the cursor's image that is active) of the cursor at the bottom of the Preview window.
The Web Preview shows your image as it would appear on a web page. At the bottom of the Preview window, enter the address for the web page you are designing and the address for the image in the web page you want to replace. Opacity will then substitute your image for the one that would normally be loaded from that address on the web page shown.
The Elements Preview lets you use your image for other preview elements. You can temporarily use it as a dock icon to see how your image looks for an application icon. You can also use it as a temporary status menu image and a toolbar image.
The Resizable Image Preview lets you see how your image as a stretchable image.