Like Sharps idea of having a low res version, some other web based mmo's have an option like this, Travian has lower res pictures but you can hardly tell the difference, when playing on 2g/3g it can be hard to load everything quickly, a seperate app however limits the other players, if the game is a success it could be moved to other phones

Probably best to tweak these settings near the end though but worth keeping in mind... I like the current building images, no idea where you got em but surely a blue and black images can be compressed far more than a full colour one?