Crystal, because your images are very large, we would highly recommend uploading the images in the app and letting the app store a copy of your photo. This automatically optimizes the images for the web and will create the second thumbnail size for you. If you just reference the full resolution images, you will find that your story is very slow to load, especially on slower internet connection. We recommend that all photos used in story maps be less than 500kb to make sure they load quickly. If you need to keep the image referenced on your server, you should create two new images (one thumbnail, and one web optimized photo) for each photo. For each of our apps that allow you to reference images by a URL, we have guidelines in our FAQ to help provide the best experience to your viewer: Frequently Asked Questions | Story Maps.
Besides just optimizing them for the web, managing them in the app will make sure they are always available when the app is available. They follow the same sharing permissions as the app and you will not have to worry about breaking links if you move your photos to a different folder on your server.
