Some of the phones that you are targetting are pretty low end, and the graphics are not sized appropriately for the screen. Each time you render the image by frame it is loading and reloading, decompressing and then drawing to the screen.
By using full screen images you are requiring the player to draw the entire screen for every frame, this is a huge waste of resources.
You should turn the images into vectors and loose the white background. Use graphics that are designed specifically for the lowest screen size to ensure that the player does not have to use scaling algorithms.