As simple as this (needs a rooted phone):

$ adb pull /dev/graphics/fb0 screenshot.raw $ dd bs=1920 count=800 if=screenshot.raw of=image.rgb32 $ ffmpeg -vframes 1 -vcodec rawvideo -f rawvideo -pix\_fmt rgb32 -s 480x800 -i image.rgb32 -f image2 -vcodec png screenshot.png