Never stretch your pictures in your sales presentations. Stretching images kills pictures impact, making them strange. Use the cropping  tools to trim and efficiently remove unwanted portions of pictures to get just the look that you want and make your documents pop.

Cropping  removes or masks areas of a picture that you do not want to be shown by reducing the vertical or horizontal edges. Cropping is often used to hide or trim a part of a picture, either for emphasis or to remove unwanted portions.

The cropping feature has been enhanced so that you can now easily crop to a specific shape, crop to fit or fill a shape, or crop to a common picture aspect ratio

How to Crop a picture

1. In your file, select the picture that you want to crop.

2. Under Picture Tools, on the Format tab, in the Size group, click Crop.

Size group on Picture Tools Format tab in PowerPoint

TIP 1:   If you do not see the Picture Tools and Format tabs, make sure that you selected a picture. You might have to double-click the picture to select it and open the Format tab.

3. Do one of the following:

• To crop one side, drag the center cropping handle on that side inward.
• To crop equally on two sides at the same time, press and hold CTRL while you drag the center cropping handle on either side inward.
• To crop equally on all four sides at the same time, press and hold CTRL while you drag a corner cropping handle inward.

4. To position the crop, move either the crop area (by dragging the edges of the crop rectangle), or the picture.

5. When you are finished, press ESC.

TIP 2:  To crop the picture to exact dimensions, right-click the picture, and then on the shortcut menu, click Format Picture. On the Crop pane, under Picture position, enter the numbers that you want in the Width and Height boxes.

TIP 3: To outcrop or add a margin (margin: The blank space outside the printing area on a page.) around a picture drag the cropping handles away from the center of the picture.

See this news in French: http://presentations-de-vente.com/impact-visuel-des-presentations-de-vente-rogner-une-image