David Pogue's Digital Photography: The Missing Manual

Straightening

Many a photographer has remarked that it's harder to keep the horizon straight when composing images on a digital camera's screen than when looking through an optical viewfinder. Whether that's true or not, off-axis, tilted photos are a fact of photography, and especially of scanning—and fixing them is incredibly easy.

Open the photo for editing, and then click the Straighten button on the toolbar/tool panel.

The minute you click Straighten, a grid appears, superimposed on your picture, and a slider appears near the bottom of the photo. By dragging the slider's handle in either direction, you rotate the image. Use the grid lines to help you align the horizontal or vertical lines in the photo.

Now, if you think about it, you can't rotate a rectangular photo without introducing skinny empty triangles at the corners of its "frame." Fortunately, iPhoto and Picasa sneakily eliminate that problem by very slightly magnifying the photo as you straighten it. Now you're losing skinny triangles at the corners, but at least you don't see empty triangular gaps when the straightening is over.

In other words, the straightening tool isn't a free lunch. Straightening an image decreases the picture quality slightly (by blowing up the picture, thus lowering the resolution) and clips off tiny scraps at the corners. You would have to view the before and after pictures side by side at high magnification ...