The Content-Aware scale command is a useful way to force your still photos to fit a specific shape (for example that of a video screen or particular print size). When used correctly, the Content-Aware Scale command can automatically adapt to preserve vital areas during the scale. You may often need to combine a skin tones option or an alpha channel to better define the areas that will be protected from distortion.
1. Open a still image.
2. Double-click the locked Background layer and give it a new name. The layer cannot be locked if you want to scale it.
3. Change the canvas size (Image > Canvas Size) to match your needs. You can also paste the layer into a new document.
4. Choose Edit > Content-Aware Scale.
5. Click the Protect Skin Tones button to tell Photoshop to attempt to preserve regions that contain skin tones.
You can also create an alpha channel if needed to protect a selected area. Create the alpha channel before invoking the Content-Aware Scale command then choose it in the Options bar.
6. Drag a resize handle on the bounding box to scale the image. Hold down the Shift key to scale proportionately. You can also hold down the Option (Alt) key to scale from the center of the image.
7. Size the image so it fits the width of the canvas. 
8. Click the Commit button or press the Return (Enter) key to apply the change.
Need Photoshop training? Check out Understanding Adobe Photoshop or Photoshop for Video.