I wonder why would you want to scale raster graphics. Scaling up will always make your images look more pixelated, while scaling down will lose some detail.
If you care about not screwing up the quality of your images, you should either settle on one fixed resolution or make it resizable in a way, that you'll simply draw more stuff on the screen (to fill up the empty space).
However, if you really insist on scaling up/down the images, then using vector graphics will be a much better approach.