If you changing your themes to use Jmagz Themes at first time. You may see that image not cropped properly on your front end. Fortunately there is plugin named Regenerate Thumbnails (we also include in required plugin), to regenerate all thumbnail.
Go to Tools→Regen. Thumbnails, and click click Regenerate All Thumbnail and wait until this process ended. It may take time base on your system speed and number of your image.