How does "Ode on a Grecian Urn" by John Keats reflect the theme that art is immortal?

The images on the urn are frozen in time. Presumably, if the urn survives the effects of erosion and decay, those images will remain there forever. Likewise, a poem (such as "Ode on a Grecian Urn") can exist forever as long as it continues to be read. There is an immortality to poetry that visual art cannot capture because the poem can be written or passed on orally. 

Keats isn't just suggesting that art is...

The images on the urn are frozen in time. Presumably, if the urn survives the effects of erosion and decay, those images will remain there forever. Likewise, a poem (such as "Ode on a Grecian Urn") can exist forever as long as it continues to be read. There is an immortality to poetry that visual art cannot capture because the poem can be written or passed on orally. 

Keats isn't just suggesting that art is immortal. He is philosophizing on what that immortality means and what its value is. The melodies he imagines from the pipes on the urn are "sweeter" because they are always there to be imagined. Melodies that are actually heard (in real life) might be flawed and only exist for a certain amount of time. Therefore, the unheard melodies shown on the urn are superior. On the other hand, isn't there something flawed about a melody that is not heard? The lovers are always almost kissing, but they never do. So, although the urn expresses immortal images and ideas, they do not "live" so to speak. 

In short, there are benefits and drawbacks to art's immortality. The urn shows a number of things that are immortal and frozen in time, but those images lack the lived experience that makes them come alive. The urn is immortal but "cold." Art, such as the urn or this poem, is flawless in its immortality, but also lifeless. How can something be immortal but not alive?

Keats ends with a cryptic line about beauty and truth. Given this discussion of life and art, he might be suggesting that truth and beauty manifest in life and art in different ways. In other words, the truth of art's immortality is not the same as the truth of an immortal (human) life, but there is beauty in both notions.