The photos and videos will automatically be uploaded in the background using the data plan or WiFi. For automatic uploads via your data plan, you have the option to limit the file size to 25MB. Files over this limit will wait for WiFi. The app’s uploader automatically pulls photos and videos from the camera, smartphone and SD card.
There will be no size limit for uploads. In case any network issue occurs, the upload will resume after the issue resolves.
Your first upload will give you an instant 500MB of free storage and for every 500MB of upload you will get additional 500MB of free storage, upto 5GB, all for free.
SOURCE: Android Police