In Java there is a command that lets you specify a source rectangle when drawing an image.
So it could just be grabbing half of the one you already have.
Or you could just cut the big image in half, 2 images.
Then on the Landscape version show both images.
So its still the same amount of memory.
When doing buttons that have states, you can put all the states in one image, left to right.
Enabled, Disabled, Mouse over, Pressed. Or whichever you want to support.
You would need to make a custom Button class that grabs the correct source rectangle.