How does John Keats use imagery in Ode on a Grecian Urn?

Imagery:

Heard melodies are sweet, but those unheard
Are sweeter; therefore, ye soft pipes, play on;
Not to the sensual ear, but, more endear'd,
Pipe to the spirit ditties of no tone

More happy love! more happy, happy love!
For ever warm and still to be enjoy'd,
For ever panting, and forever young;
All breathing human passion far above,
That leaves a heart high-sorrowful and cloy'd,
A burning forehead, and a parching tongue.