Images on your computer are sized in pixels. If the size of your image extends beyond the width of the content area it can mess up the layout of your site.
Your pictures should be around 800 pixels wide (the width of a 15″ monitor).
Images larger than that should get resized with whatever image editing software you have on your computer.
Simply open the image file in the editor, crop the image to your desired viewpoint, then reset the image dimensions. Now re-save the image.
Perhaps you should give it a new filename if you would like to keep the original image in it’s full size.