how I can translate a JPG to RGB (R:G:B=5:6:5),and display on LCD. Where I can get the source code relational.
There are some heavy maths involved! You could start searching at http://www.jpeg.org/public/jpeglinks.html
Any image manipulation program ought to be able to do such a conversion. One such where you can peek at the source code: http://www.gimp.org/ If you need your microcontroller to do this during normal operation, you'll probably need to weigh the cost in computation to decompress the JPEG image against the cost in space to store the images in some RGB format (possibly compressed) to start with.