Add images to Images.xcassets

[schifano]

Adding images to the image assets catalog helps keep track of all images. In general, it's good to add any UI related images here. In the future, it could also help scale the images as device sizes change. For example, it is possible to use vector images and let Xcode generate 2x, 3x, etc sized images at run-time. http://martiancraft.com/blog/2014/09/vector-images-xcode6/