How do I resolve photos that are marked: "Low resolution alert: [user] hasn't finished uploading this photo"?

 You may have closed the app before all of your photos have finished uploading. Just open the app again on your phone and the uploading should restart and the full sized images will be added.

Be aware that there is no "background uploading"—meaning the app has to be open for the uploading to occur. If you're connected to wifi, make sure you have a strong and consistent connection.