I am trying to understand the reason for having multiple image sizes. I know there are different size screens with different densities, but wouldn't it be acceptable to just have the largest resolution image and load it to the size you need rather than have lots of different images?
Scaling an image takes considerably more processing power than just loading a pre-rendered one. Also, a naive/simple image scaling algorithm would look pretty low quality compared to one that had been actually properly rendered at the right size in the first place.