You cannot make the game download assets when needed (it will download all at once), however, C2 create an offline.appcache, this file makes the app totally offline after downloaded for the first time (it needs a little time to store everything permanently after the game is launched though), and so your app will load instantly next time.
However keep in mind that the update to new version of the app should be taking care off, More informations about this can be found.
For your images, try to not have them too big when not needed, also you can change the compression to lossy PNG-8 on some images, could save so download size.