Metric:

(image_decode_width * image_decode_height * bits_per_pixel)
 / (image_screen_pixels_width * image_screen_pixels_height * bits_per_pixel)

image_screen_pixels_width is the width in pixels of the screen area
that the image actually takes when drawn. ditto for height

bits_per_pixel is the pixel bitdepth. This will vary on low-end
devices.