When you hover over an image you will see that a blue line will surround the image and provide you with a button to click, which will allow you to edit it. Once you click the edit image button, you will be able to upload an image, zoom, rotate, name the image, link to another page or video and delete the current photo.
On this page you will also see that there is information regarding the ‘optimum image sizes’ this gives you a guideline as to what will look best in terms of pixels. (See photo below).