Will Apple reject an app with "background fetch" which is used to upload a photos
Background Fetch only allows you to download data and the process exit if the transfer time is superior to 30 seconds. There are limited things you can do while the app is in Background mode : Background Execution

If you want to upload your photos in the background you'll need to use background transfer service, but once the task is finished the app will not upload more data. This tutorial explains how to download a file without the application in the foreground but you can use the same logic to send data : iOS 7 SDK: Background Transfer Service