Sometimes your image does not fit fully into the page you are editing or y just don't want the entire image to be seen. In those cases you can modify the area that is visible by simply double clicking the image and than moving the focus area. This will in effect crop your picture while keeping the ratio.
To do this first double click on the image in order to open the crop screen. It looks like this:
The grayed out area on the bottom and on the right will get cropped out. TO change the cropped area simply move the icon seen in the bottom right corner.