The default setting for an image is for it to Fit in the bounding box. When an image is set to Fit, the entirety of the image will be contained within the bounding box of the image. The image will also maintain it's aspect ratio, regardless of the size of the bounding box.
When an image is set to Fill and Crop, the image will expand so that the entirety of the bounding box is filled by the image. This will not change the aspect ratio of the image.
You may notice in the center of your image there is a green and black circle. This circle is your image Focus. Using this, you will be able to control what part of the image you will be looking at, allowing you to Crop your image.