I have tried resizing, different formats, resetting, clearing cache and cookies. No matter which photo I place it shows as a gray box with red exclamation point.
Which browser are you using? Have you tried using a different browser?
Is this always the result, or just today? It may be that your internet network (or maybe the Express servers) is having trouble uploading your files.
Have you double checked that your photos are using the correct file formats and size restrictions? Express only acceps jpg and png images that are less than 40mb.