Skinning and Image size for android devices

I am building a mobile application for both Android and iOS devices. I am using images in 3 different resolutions sets.

The images have been made in such a way that the complete all screens were first designed for 640x960 resolution with 320 dpi. Then I created 3 different sets of images where the images were reduced to 75% and 50% of the 640x960 image size. The images I have now work fine for iPhone 3GS and iPhone 4 and 4S devices. But the 75% one's don't work correctly. This is because the size of say background of view at 75% would be 720 pixels while the size of the Android device's screen is 854px or 800px.

How can I address this issue?