Once you have selected a photo product, you can upload a photo by clicking on the photo upload button, which will have an upload symbol on it like this:
Then you will be able to choose whether you upload it from Facebook or your device. For more information on what kind of photos to upload click here.
Once you have chosen a photo (or photos if you're creating a photo book), this can be uploaded to your library, where you can add a filter, rotate or scale (i.e. zoom into) the image. To change the photo, just click on the upload box again.
Please note - the shortest side of the photo will automatically fit to the respective side of the box, which can cause "cropping". This is because the image can only be zoomed into, not out, so that areas of the box are not left blank.