The recommended image size varies depending upon what the intended usage of the image is.
Front page image/slideshow (described here)
Images for the front page slideshow must be exactly 885x432 pixels in size.
Leading Images
Many content types can have a "Leading image", which appears at the top of a page as illustrated in the following screenshot.
Ideally, these images should be 885 pixels wide and no more than 432 pixels high. An image less than 432 pixels high will display without problems, but it is best to try to be consistent across the site for aesthetic purposes.
Images embedded in page content (described here)
By default, images included in the main page content will automatically resize to fit the screen size of the viewer. It is recommended that you do not upload image files which have a large file size, as this make the page slow to load, so it is recommended that you resize your images to have a width no greater than 885 pixels before uploading them.