Appcelerator filesystem and deploying data files at build time

I need to install an iOS and Android app built with Appcelerator Alloy which contains a large number of photos (about 200Mb in total). From time to time the user will be able to download new photos from a remote server, but only at the rate of about 10Mb each time. I have built the app and it works fine using the Resources directory, but I now realise this can only be read (not deleted or updated) by the app on devices. I am not clear about where in the project filesystem I should put the photo files so that they will be in the ApplicationDataDirectory when the app is installed. Do I need to somehow move/copy all the data from the resources directory into the ApplicationDataDirectory at build time? Seems a bit long-winded.... Sorry a bit of a long question, but can't find an answer anywhere! Any help or advice much appreciated.