By default, user-uploaded images are set to fit the area of the placeholder. If you want user-uploaded images to fill the entire area instead, you should enable image cover for that image placeholder.

Here's the difference between images uploaded to fit, and images uploaded to fill the placeholder area.

The image on the left is set to fit (default behavior, with image cover disabled). Here, the uploaded picture is resized to make sure the entire picture fits inside the placeholder. Depending on the proportions of the picture, some areas of the placeholder will remain empty.

The image on the right is set to fill (image cover enabled). Here, the uploaded picture is resized to make sure the picture covers the entire placeholder, where no areas of the placeholder are empty. This is also referred to as auto stretch.