Portrait images are great for staff photos or when an image is taller than wide. They work wonderfully when displayed to the right or left of text to highlight an individual or specific object.

Many times a square image will do fine to showcase a specific object in an image. This format allows the image to be pleasing to the eye due to it's symmetry.

Standard images tend to be what is used most of websites. It's simple to create, many cameras are already at this particular ratio and shows most of the image without the need for resizing or cropping.

Typical wide format image, heavily used on the web to break up paragraphs in a long article or as stand alone images. This is great when displaying images displaying a horizon line or wide objects in the image. This is also the typical format for videos on the web.