Properly cropping your images is a basic requirement for publishing content online.
More information on correct image display in Metro Publisher can also be found here: Working with Images.
Too Much Whitespace
Often, other page elements will align relative to your images. If the margins within your images are not correct or are uneven, adjacent page elements will appear mis-aligned. For this reason you should avoid adding unnecessary whitespace to your images.
Example 01: Improperly Cropped Image
This image has unnecessary and uneven borders.
The result is that the image does NOT fit properly within the page and does NOT visually align with the other page elements.
Example 02: Properly Cropped Image
This image is cropped properly and has no unnecessary whitespace.
The result is that the image aligns properly with the other page elements.