If you’re using OS X 10.4, you probably already know that Preview has some nice new features—such as being able to change the sort order of a folder of images by control-clicking on the white space in the image drawer. You might also know you can use Preview to copy-and-paste a portion of an image. Just start dragging with your mouse, and you’ll see a box appear. Keep dragging until the box has highlighted that portion of the image you wish to copy, then press Command-C to copy the selection. Press Command-N now, and Preview will open a new file with the contents of the clipboard.
But if you’re trying to use Preview to snip a portion of a larger image—say you want a 640-by-480 pixel selection out of a 23-inch 1,920-by-1,200 wallpaper—it seems you have to guess at the dimensions of your copy box, as Preview doesn’t show them. Well, it turns out that it won’t show dimensions by default, but it will show them if asked. All you need to do is press and hold the Option key before starting your drag, and you’ll see the width-by-height pixel dimensions of your box. This simple movie shows the differences—first without the Option key held down, then with:
While it’s clearly no substitute for a “real” graphics program, you can use Preview (in 10.4 at least) to handle simple editing needs—and knowing the size of a dragged region is something that definitely helps with those tasks.