You may already have a clear idea of where you want to display the images you select and it is important to get a feel for how they will look on display. Particularly You need to understand how size selection and cropping can affect an image, particularly if you are looking at grouping images together. My cropping guide below shows how both landscape and portrait images are adjusted to give you an idea of how size selection can enhance an image when it is done right. The greyed areas are lost in the images below when printed at the sizes listed. 

The size of an image can impact on how it is perceived and there are a number of rules of thumb that are used as guides to composition. The most straightforward one to consider is to position a point of focus on a 'third'. If you imagine the image divided into 9 by two vertical lines 33% in from left and right, and two horizontal lines 33% from top and bottom, then the points of intersection are regarded as being generally pleasing to the eye. Taking this into account when choosing image size and cropping can help to ensure the best result for your final images.