Dropbox for iOS (not sure about Android) does not allow the user to export multiple photos to the camera roll of the mobile device. This is only achievable one at a time by opening the photo and then exporting from within the sub-menu.
At present there is a option to select multiple files within Dropbox folders but the only resultant function of this is to 'move' files once selected.
I would suggest that a feature to export multiple photos to a camera roll at once is added. The get-around would be to activate auto photo sync which is not an ideal solution for people only wanting to export certain batches of images.