When on the Photos page while editing a project, you can drag images from one photo slot to another to reorder:

Note: If the option to automatically crop photos for this project is enabled, the photos will automatically be re-cropped when you move them from one slot to another.  If the option to automatically crop photos for this project is disabled, you may need to re-crop one or more photos after moving a photo from one slot to another. See "How can I enable or disable the automatic cropping of images?" for more info.

Use the Preview Project button at the top of the left column, to see where each photo will appear in the final project.