Q. "Can you pick and choose what to back up, or does it just copy everything? I have more pics than will fit on this, so I want to select what I back up."
Yes, you can select particular photos or even albums to back up. Or you can select to back everything up at once. If you select Backup Everything, then it COULD take a long time to back everything up. For this reason, if you have a lot of photos, we recommend that you start with a full charge so you don't run out of battery before the phone turns off.
Even so, if your Backup is not complete before the phone powers down or when the phone is recharged, you can simply restart the app and it will pick up where it left off! And for most people, even those who have tens of thousands of pictures and videos have not filled a device with 256GB.