Does Dropbox use local phone storage when set to display photos in the gallery? As the title suggests, I would like to know if syncing my Android Gallery to Dropbox and changing the Cloud Dropbox setting to "Pictures - View pictures from Dropbox in Gallery" will use local storage space?

For example, I can set it to view my Dropbox images in my Gallery. However, if there photos are already on my phone, will this take up twice the space? OR does it pull the photo as and when required? 

Another example, If I have a new phone with no storage being used and I set this option, will local storage space be used to view these images?

The reason I ask is because my wife is running out of space fast on her Android device and I have suggested using Dropbox to store all of the photographs and videos but she would like to still see these in her Gallery. Would this resolve the issue or would it use all the space again when I enable this setting?
The photos will not be stored doubled. They will be downloaded every time you want to see them (and may be temporarily stored in cache).