Added placeholders for images in the app.

These will be updated when I get an opportunity to change the sizes.

NOTE:  xxhdpi: x2.5
        xhdpi: x2.0
	 hdpi: x1.5
	 mdpi: x1.0 (baseline)
	 ldpi: x0.75 (don't support, android scales properly)
Source: developer.android.com/training/basics/supporting-devices/screens.html