Preview is Mac OS X's built-in graphics viewer, fax viewer, and PDF reader. It's always been teeming with features that most Mac owners never even knew were there—but in Leopard, it's become a real powerhouse.
Preview's hallmark is its surprising versatility. It can display and manipulate pictures saved in a wide variety of formats, including common graphics formats like JPEG, TIFF, PICT, and GIF; less commonly used formats like BMP, PNG, SGI, TGA, and MacPaint; and even Photoshop, EPS, and PDF graphics. You can even open animated GIFs by adding a Play button to the toolbar, as described below.
If you highlight a group of image files in the Finder and open them all at once (for example, by pressing ⌘-O), Preview opens the first one, but lists the thumbnails of the whole group in the Sidebar. You can walk through them with the ↑ and ↓ keys, or you can choose View → Slideshow (Shift-⌘-F) to begin a full-screen slideshow.
Use the menu below the Sidebar group to change the sorting order of these thumbnails.
To crop graphics in Preview, drag across the part of the graphic that you want to keep. To redraw, drag the round handles on the dotted rectangle; or, to proceed with the crop, choose Tools → Crop. (The keyboard shortcut is ⌘-K.)
If you don't think you'll ever need the original again, save the document. Otherwise, ...