Handle max_kb in nokia.image-processing localmsg api

I tested with several images, the size of generated image is about 10%-20% larger than the given size which I think it's acceptable.
I wonder if there's a good way to estimate the size. According to http://en.wikipedia.org/wiki/JPEG#Effects_of_JPEG_compression, with a quality of 100 the image would be encoded roughly with 9 bits per pixel.