For every image of your site, you can choose wether to show the entire image or just a part of it, by cropping it.

You can crop the image via the canvas that appear on it when you click on it, defining the area you wish to be shown within the block.

From the options of the block the image is located into you can also decide if, when clicked on, the image must be shown zoomed whole or if the zoom has to be applied only on the cropped part.