You can use the cropping feature to cut out unwanted parts of the image to emphasize the subject in the picture.

Cropping an image to a specific aspect ratio

1. Open the image in the View & Edit area
2. Choose Image > Crop Special to bring up the dialog box
3. Select an aspect ratio or enter a custom aspect ratio inside the Constraints group box
4. Drag one of the eight handles to resize the cropping area, darkened area will be cropped out
5. Drag inside the cropping area to move the cropping area
6. Press OK to finish

Cropping an image with selection

1. Open the image in the View & Edit area
2. Make a selection inside the image
3. If the image selection is what you want to keep, choose Image > Crop, or click this toolbar button 
4. If the image selection is what you want to remove, choose Edit > Delete

If there are holes in the image after cropping, you can fill up the holes by using the Image > Add Background Fill command. The Image Documents pane on the left of the main window shows the current background fill color setting. You can click the color square to change this setting, or click the bucket icon to add the background fill to the image.

You can also use Image > Fit Canvas to Image or this toolbar button  to adjust the canvas to the size of the image after cropping.