You probably don't want to use subviews for this. There is a CALayer
subclass for exactly this purpose: CATiledLayer
: https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/CATiledLayer_class/Introduction/Introduction.html
With it you can even load different image tiles based on how far you are zoomed in, just like Google Maps. It will render without borders and the performance will be WAY better than using lots of subviews. Subviews are expensive, layers are (generally) cheap.
Update: This example will get you up and running: http://bill.dudney.net/roller/objc/entry/catiledlayer_example