Dropbox Carousel allows you to backup photos and videos using Wi-Fi and cellular data, or restrict it to only use Wi-Fi network. You can choose the Wi-Fi only setting to save some bandwidth and backup photos. If you are using Android device, then Carousel backs up your photos in the background without open the Carousel app.
If you are using iPhone or iPad, then you have to open the Carousel app to start backup process. It will pause after a few minutes when your iOS device becames dormant. If you are backing up a bunch of photos on your iPhone or iPad, then you can re-open the Carousel app at any time and check the progress of your photo backup. This is due to a time limit iOS puts on apps running in the background. With background uploading, Carousel will automatically resume uploading for a few minutes whenever it detects a change in your iPhone's location.
If you choose Wi-Fi only option for Camera Uploads, then background uploading will not use your cellular data. It will resume when your device connects to a Wi-Fi network again.