Image Assets can be displayed two ways, called Contain Mode and Cover Mode:
Contain Mode will always keep the original aspect ratio and won't be cropped. It's normally used on images like product shots or logos.
Cover Mode will have the image cover the entire size of the element box. The image might be auto-cropped if the element box's aspect ratio is different from the image's. The center of the image will be where its focal point is.
Here's a quick animation of both modes, and where the option to change it is located?