To which noun does a pronoun refer?

Think of the sentence this way: "This black and white photo (was taken when 67p was approaching the sun and) reveals a jet of ice erupting from its crumbling surface.

The black and white photo reveals a jet of ice erupting from its crumbling surface. Now we go back to the part in parenthesis to find out what "it" is referring to. What is it a photo of?

It is a photo of the comet as the comet approaches the sun. So the "it" is the comet.

This black and white photo was taken when the [probe] voyager was approaching [the comet] p67 and reveals a jet of ice erupting from its crumbling surface.

The "it" is still clearly referring to the comet. The comet is the noun (the antecedent) closest to the pronoun "it". Generally, antecedents are the closest noun to the pronoun.