If you need to extend a photo by adding more sky, clouds, grass, snow or other organic "background", here's the easy way, using Photoshop CS5 or later:
1. Choose Image > Canvas Size. Increase the width or height of your image, choose the anchor, set the Canvas extension color to White, and click OK.
2. Choose the Quick Selection Tool (W) in the tool panel. Be sure that "Auto-Enhance" is selected in the Options bar at the top of the screen. Then select the additional white canvas you added to the photo by dragging through it with the Quick Selection Tool.
3. Press the Delete key.
4. In the Fill dialog box, select Use "Content-Aware" for the Contents. Click the OK button.